Because it could eventually be used to keep us in, a-la East Berlin.
Before the usual suspects chortle at this possibility, I'm reminded of all the Bushbots who poo-poo-ed the warnings that provisions of the Patriot Act, NSA spying, and the idiotic FISA "court" could be abused as political weapons.
If you want immigrants who come here not to be burdens upon the system, better that you finally pare back the socialistic welfare state like you've been promising to do for decades.
This wins the whole fucking board in my opinion.
Free nations don't build walls. They build bridges.