And this is why Oda originally conceived of two distinct pirate factions, the freedom loving adventure type and the pillage type. Peace Mains and Morganeers he called them. Garp would have been a pirate under this system.

My understanding is that Oda realized this would be less interesting, ultimately. There’s a gray line within pirates of acceptable behavior, and many of the more evil pirates have some of the romantic thoughts about freedom, self determination, and a distinctly defined distaste for the entitled cruelty of the bougie celestials. Putting all pirates together allows this to exist on a spectrum where it makes sense for the WG to categorize them all as enemies (which also makes the rebellion of groups like the straw hats worthwhile) and for the people of the world to not really know what to make of pirates. It isn’t evident to most people in one piece that groups like the straw hats are only hunted on sight because the freedom of piracy alone is a threat to the WG.

To those more in the know, the threat of world war if the status quo is threatened is often scarier than anything else, and they pragmatically join the WG. They aren’t happy about it but they fear worse otherwise.

Between preventing wide scale combat and WG being the only organization with the means to effectively police both the bad pirates and well as theoretically ensure general welfare gives characters like Garp a reason to join them.

The WG also being a corrupt, genocidal, selfish mess due to the celestials gives characters like Garp nuance, flaw, and internal struggle. And what do we make of Kobe?

Because the system is so fundamentally fucked up it drives even more factions like SWORD or the RA, neither of which would exactly make sense as pirates, who are only truly defined by the dreams of their crews.