States are still free to pass laws in matters that are not explicitly granted to fed.gov in the Constitution. For example, the Constitution is silent on drugs (Prohibition was repealed), kidnapping, and various other matters which are left to the States. That state laws duplicate, in many instances, Federal laws, is irrelevant to state prosecutions.
Holder seems to say that immigration policy is one of those areas constitutionally reserved exclusively for the Federal government, and that if they won't enforce the laws, nobody else can, either.
If this "reasoning" is upheld by the courts, then in the future problems may arise if a state hands over a case to the Feds . . . if a State is enjoined from enforcing Federal laws, then it seems reasonable that states would be enjoined from even assisting in the implementation or enforcement of Federal laws and requirements.
This would mean no state, county, or local backup when FBI, DEA, OSHA, or any other alphabet agency stages a bust, raid, or even an investigation. The State could still enforce it's own laws, but co-operation with the Feds operating locally could get dicey.
AND . . . if a court throws out the AZ law based on Consitutional exclusivity . . . look for a LOT of 10th Amenndment suits against fed.gov, citing that as precedent, to overturn a LOT of Federal laws.
The only thing that seems certain is that a lot of lawyers are going to get rich.