Uh, they aren't TAKING the money. They are being GIVEN the money by people who are illegal. What do YOU propose they do with it? Since the people GIVING the money are lying about who they are, how to you propose they track down the rightful people and credit them in some way or give them the money back on their way back to Mexico?
Personally I agree with the idea of giving it to the schools, hospitals and welfare systems which are also being used by these fake SS numbers.