But as an illegal alien you aren't supposed to be here in the first place. This isn't like the availability of a product for legal citizens (alcohol). This is about restricting the wiring of money by illegal aliens to foreign nations. Should we change the laws and not crack down on terrorists who try to wire money to one another? We go after those funds transfers, but by your logic we could say that those who seek to go around the law will do so anyways. That just seems like a defeatist attitude to me?
During prohibition, alcohol wasn't supposed to be in the country, either. But there was a market for alcohol, just as there is a market for illegal labor.
My bigger point is that measures like this are aimed at hurting the supply of illegals. But we'll never fix the problem unless we reduce the demand for illegal labor. As long as the demand for something exists, there will always be a supply.