It has more to do with pandering for votes than with any economic concerns. What they would pay in Payroll taxes they'd get back several times over in earned income credits and a whole host of social welfare programs.
Moreover, if you have an employer who hires illegals and pays them sub-minimum wage (i.e. no payroll tax) under the table now, that same employer won't change his practices just because the Federal Government made the same workers citizens by decree.
I don’t know the number of votes involved is big enough yet. While you’ll be paying out with welfare etc. for the new citizens, you’ll still get back more from them than we currently get from the permanent underclass they are to replace. As for employers that think they’ll continue with business as usual, now their citizen workers will turn them in and sue them as well.