I think the point here is: why do we even have a law against them working here? Because people don't want them to be undercutting American workers and creating a whole sector of cheap, desperate, disposable, underqualified laborers. Just punishing employers who disregard safety laws, fail to check workers' backgrounds, and underpay people would seem to address that issue at least as well as punishing the laborers themselves. Punishing illegal immigrants is likely to actually exacerbate the problem since illegals are less likely to complain to the police about workplace/pay violations.

"Punish them because it's illegal" is sort of like saying "I'm opposed to legalizing marijuana because it's illegal and associated with organized crime," instead of talking about legalization's effects on public health.