There's no problem with mobility of human capital provided you don't leave the food dish out and allow those who are mobile to steal the food that they did not have any part of creating. If said immigrants get nothing they didn't help create themselves then and only then will they come here for reasons of putting in hard work and upward mobility rather than the parasitic behavior they display now.RTWT. As if we don't have more than enough native-born parasites?
What you can't do is support both a state "safety net" that is present (or larger) in one place while it is absent (or smaller) in another and at the same time have open borders.
If you do this, and we have been doing this, you don't get migration for economic opportunity, you get locusts who strip the land raw -- people who come to your nation as thieves, not participants.
Let's see...we're saying, "Come here--legally or otherwise--and we'll give you everything and ask for not one single ounce of effort in return. You don't have to get a job, learn our ways, respect our customs, follow our laws, speak our language or assimilate in any way. You'll get free food, shelter, money, medical care, cable and a cell phone, all for contributing absolutely nothing."
Gee...what could possibly go wrong?