Which does not contradict Federal Law. What happened was the the Feds chose not to enforce that law so Arizona made it a state law and told the Feds that they could not stop them from enforcing Arizona law. The Courts said yes they can because the Supremacy Clause prevents end-arounds of enforcement policy like that.
And now the feds are also going after cities that explicitly don’t cooperate with federal deportation policy:
So the bottom line is that the feds find themselves to conflicted to do anything substantial for the last half century, but they pounce like cats when anybody else tries to make a move.
So…incidental gridlock, or intentional stalling?