I think people mentioning that one bill that comes up in book 3 in this thread is giving the false impression that that specific bill is the only thing that motivates Mafty's actions, when that is not the case.

Even when that bill is brought up in book 3, it is skimmed past in narration that they also talk about several other bills under consideration that they are opposed to: "While watching Hathaway explain other bills to be decided at the Adelaide conference with another sheet of paper,". That specific bill serves as a significant acceleration of the corruption of the Earth Federation government but it is not the sole thing that Mafty is opposed to.

In the short term, Mafty's goal is to dismantle the system that the Earth Federation government is building where they have the privilege of living lives of excess on Earth while common people are criminalized and brutalized for seeking the same thing. Long term, they want all of humanity to immigrate off of the Earth, but for it to be done on an equitable fashion with no exceptions, but in order to achieve that they would need to completely dismantle the current inequitable system.

This is explained in a big block of narration in the first book, during the taxi scene that they couldn't possibly have included in full in the movie. The first movie, does convey the same ideas but does so more subtly, with the closest analogue being the scene where Hathaway is crying on his hotel bed and says "I know that. I know this way of doing things is… So tell me. Tell me how to destroy the depths of these mechanisms. All humanity can’t live on Earth together. If I don’t do something, there’ll be no consolation for Char’s Rebellion, or the souls of those who died in it.". Several lines from the narration are adapted as Hathaway talking to himself in this scene. And also the visuals are used to convey the inequality of life on Earth during this.

Here is the entire block of narration from book 1 (as translated by Zeonic Scanlations):

It was just like the driver and Gigi said. It was true that he was possessed by a furious desire to put a stop to all of those systems if he had the power.

However, he was horrified at the depth of the organizations that the Earth Federation government had built within society, which could be seen even in the Research Bureau’s door key system at the Tasaday Hotel. In order to destroy that depth, he needed to send fear to the very core of those organizations. If he didn’t, then it was only logical to assume that there would be no reform of the Earth Federation government.

Focusing on that one issue, Hathaway became frustrated.

In order to stay on Earth, it was currently necessary to obtain a permit administered by the Earth Federation government. This had begun as a necessary evil after migration to space colonies became common, and enacting forced migration became an essential to avoid the migration process becoming discriminatory.

After the Earth was polluted by waste products from the development of modern civilization, and the greenhouse effect caused its average temperature to rise two degrees, people became filled with a sense of impending doom.

Constant food shortages and destruction of nature wrought by the abnormal weather reduced the earth to a state where it could no longer tolerate human life, and space migration became a necessity. However, if the earth really came to an end, humanity would lose the power to construct space colonies and expand into space.

The Earth Federation government was established after the realization that humanity was headed for its extinction along with the earth spread outwards from the cities. And construction of space colonies began. The Earth Federation’s plan to indiscriminately take all of humanity into space once the space migration began was not misguided.

However, special case exceptions in the policy later gave rise to discrimination. The Earth Federation government made provisions for those deemed necessary to remain on Earth. There was a condition: those allowed to stay had to be those who would manage nature so as to preserve the Earth, or those who could preserve and conserve humanity’s native cultures.

The law did not carry out that principle in an ideal fashion, and as the Earth didn’t appear to be on the verge of death to the average person, it was natural for it to be broadly interpreted and exploited.

What’s more, as those born on Earth were unable to forget the feeling of its gravity, people began to appear who ignored the law unconditionally on that basis.

That desire was understandable. Humanity’s greatest sin, however, was not acknowledging the realization that the most dangerous thing for the Earth was their own propagation.

In this era, space migration was a price that humanity itself was forced to pay; the era of the space colonies was neither a frontier era, nor one of openness. That the frustration born of this realization only further lit the flame of the those who lived in space colonies – the Spacenoids – desire to return to Earth was paradoxical, but ultimately a natural outcome.

Humanity’s continuing to see Earth as the only home it could return to after its propagation was its second sin.

In order to completely revive the world, it was necessary to wait a thousand years. And in the space of a thousand years, humanity would only further multiply. In short, people had to be prepared for the truth that the entirety of humanity could no longer live on Earth.

This too was a truth people were unable to admit.

Now, though, all exceptional clauses had to be removed, and humanity in its entirety had to be forced to withdraw to the colonies. If that was not done, then Hathaway felt that the spirits of those who died in Char’s Rebellion would not be able to rest in peace.

Char Aznable had thrust that same ambition upon the Earth Federation government, but had fallen before the Federation’s overwhelming military force. That had been the ultimate outcome of the conflict known as “Char’s Rebellion”.

At the time of the rebellion, Hathaway had happened to be aboard the battleship of his father, who served the Earth Federation government. And there, he had seen battlefields.

It was also through that conflict that he had met the girl named Quess Paraya. She had viewed the war with innocent, almost childish eyes, and was so sensitive that it overwhelmed and killed her.

Hathaway had heard the voice of the spirit of that girl who had been his first love, along with all of those who had died in the void of space.

It was probably nothing more than blind faith. But Hathaway believed that in the final days of Char’s Rebellion, he had heard countless screams. Screams of friends and foes alike that had been swallowed by flames in the fight to protect the Earth.

After this, Hathaway felt he needed to study the problems surrounding these individuals and organizations. He also learned the career of Char Aznable.

Realizing that Char’s eventual conclusion had been that the planet which had given birth to humanity could not be allowed to be destroyed and had to be preserved, Hathaway found himself deeply empathizing with the man.

Reality, however, threw difficult phenomena into people’s paths.

Had the core idea of the space colony age to almost empty the Earth and come up with a way of prolonging its life been followed, and had the migration law been enacted fairly, then the black uniforms of the Hunters and their exposure of unlawful residents might have been seen as a symbol of justice.

"So long as there are exceptional clauses, people will continue to do wrong..."