Having a law is one thing. Enforcing it is quite another.
Nobody can enforce any law unless the very first people who come into contact with a criminal take him into custody and then follow the legal procedures after that. The problem is that the police are usually (certainly, in the case of NJ) prevented from either enquiring about immigration status or notifying the federal authorities of it, so therefore this stuff never even gets to the enforcement phase.
The bill before Congress actually did include a provision to punish states and localities that forbade their employees - law enforcement, medical, welfare, driver’s license, etc. - to cooperate with Federal law. The bill (which I thought was lousy on other grounds) didn’t get passed, but this is something that should be part of any other federal efforts.
I love the Mayor of Newark going around emoting when his very own system forbade this guy’s being reported in the first place - and a local judge knowingly gave the killer bail.