This can't be this hard for you to understand:
Justice Department lawyers argued that the state statute should be declared invalid because it has improperly preempted federal law.
"In our constitutional system, the power to regulate immigration is exclusively vested in the federal government," the brief said. "The immigration framework set forth by Congress and administered by federal agencies reflects a careful and considered balance of national law enforcement, foreign relations, and humanitarian concerns -- concerns that belong to the nation as a whole, not a single state."
In this case, the Feds claim that the state is preempting federal law. In San Fran, they make just the opposite argument. That San Fran is free to ignore Federal law (detainers) since it is best to leave this decision up to the local authorities.
Have you thought that just maybe the reason the Federal government jumped in on the Arizona situation was because of the possibility of racial profiling? San Francisco didn't have that situation. That's how I take it. But I have thoughts outside of "everything is Obama's fault" that some have.....