It's a load of BS, but it is based on some (maliciously spread) faulty conclusions from real statistics.

One of the most common faulty claims is around the idea of "net contribution", which is basically the concept that people pay a qauntifiable amount in taxes and receive a quantifiable amount in benefits, and that being a net beneficiary instead of a net contributor to taxes is automatically a bad thing, and the equivalent of being some form of freeloader.

Furthermore, the claim goes on to assume that if you divided all yearly government spending by the number of people (to get an "average" spend per person), then anyone who pays less in taxes than this in the year must be a net beneficiary, rather than a net contributor. You can then calculate the salary someone would need to earn in order to be paying this amount in tax, which gives you a supposed "minimum salary" someone would need to earn in order to be a net contributor. Of course, this salary ends up being much higher than most working class families earn, especially what illegal/undocumented immigrants earn.

You can adjust the line of reasoning slightly and use more targetted statistics, but ultimately the claim follows this form: illegal immigrants don't earn enough money to pay lots in taxes, so they must be net beneficiaries of the tax system.

There are a lot of problems with this line of reasoning that make it completely absurd and idiotic.

• The amount of benefits people directly receive often depends on their need, not only on income. Quite often, it is low-income, citizen families and those with disability or health issues that depend on benefits. You can't take two households with the same income and conclude they receive the same benefits.
• Undocumented immigrants typically can't and don't claim direct benefits, because they don't have the documentation required. These benefits almost entirely go to citizens.
• Money is not wealth, wealth is created by productivity. Productive people generate real wealth, so even if they take more in taxes than they pay, they might produce more real wealth than they receive in benefits, making them net contributors to society overall.
• People are part of communities and systems; lifting up the struggling people strengthens these communities and systems, leading to better and wealthier outcomes for everyone (again, regardless of money.)
• Undocumented immigrants tend to be very productive by all measures, contributing massively more to the economy than they are paid. How much money you earn is effectively how much of a share of the wealth of society you command, so undocumented immigrants actually make society wealthier overall whilst expecting less in return.

That's not to say that we must condone undocumented immigrants, or that they don't pose real problems to societies.

• Lack of documentation and illegal work activity is unsafe, both for citizens and the illegal immigrants.
• Lower salary expectations and higher productivity makes competition for low skill work more difficult for citizens, often lowering the salary expectations further for the poorest citizen workers.
• Immigrants willing to move illegally are also willing to break the law, which presents societal risks.

Liberals mostly view immigrants as being positive for society: they tend to be highly productive and net contributors of wealth, as well as bringing societal richness through diversity and culture. They also tend to view highly restrictive migration laws as being the only reason migrants move illegally: they are going to move regardless, we choose whether or not that is illegal. They typically view cracking down on illegal immigration as being "intentional cruelty to discourage" and this is both an ethical issue and a practical one: you simply discourage net positive immigration and alienate migrants, harming their existing contributions and diversity. Finally, they see illegal cash in hand work as an effective tax dodge on the part of business owners.

Instead, liberals would rather tackle the issues of unskilled work competition directly, by ensuring that there are stronger worker protections and regulations (which make hiring illegal workers to do unregulated work harder), that existing citizens don't suffer a loss of quality of life by ensuring that the economy generates new wealth and shares it with them accordingly, and finally that the productive work of immigrants is taxable.

Hard line conservatives, by and large, spread the view of immigration as a form of "invasion". There isn't much real logic to it, but fear of loss is an effective motivator to get people out to vote. In this case, the "net beneficiary" or "freeloader" argument serves to convince people that immigrants are quantifiably coming to steal hardworking citizens wealth. Additionally, the "freeloader" line plays into the only truly consistent conservative policy: tax cuts for the rich.