You ask good questions. In brief, I think the immigration laws should be enforced. The fact that the number of illegal immigrants in the US is so large that enforcement is nearly impossible makes this problematic.
They'll sneak back in anyway if they're deported, so what's your suggestion?
They may not sign up, but the policy that we're not going to slap them in chains and cart them to prison may be incentive enough to come out from the shadows.
See post 268 for some of the things that I think should be done.