OK, there's technicalities for police to do ANYTHING to help the Feds when it comes to immigration, if you want to get all criminal defense lawyery about it (made even more difficult with sanctuary laws). I'll admit part of your argument.

Doesn't seem to matter if a LEGAL citizen breaks a federal law, police hand them over no problem all the time. Illegal immigration is a federal law, why the prejudice against it, specifically? I guess I could demand the same rights if I broke a federal law then, if in local police custody, right?

If it doesn't seem to matter in states without Sanctuary laws, why do some cities and blue states feel the need to have them, if it doesn't really make any difference, legally?

They are basically shutting out or at least handcuffing a federal jurisdiction, using states rights as an argument for it.

Surf 'em if you got 'em