Please read my friend Holger's analysis here: U.S. Border Patrol Union Joins ICE in Ridiculing Obama and Holder.
The money quote is
So let's call a spade, a spade. Obama and Holder's challenge to SB 1070 has nothing to do with immigration in America - it's about Federal control over the individual states and their sovereignty. This probably wasn't Obama's first choice of when to have to put the states in their place but he was forced into it. We have to realize that once there are some state victories against the Feds, whether it be on immigration or healthcare or something else, it will be like a dam bursting and in Obama's twisted mind, he's racing to bankrupt the states as soon as possible so that they have 100% dependency on the Federal government.