Personally, I don't believe that the ATF is overstepping any bounds. Federal law prohibits the use of marijuana, medical card or not. Just because it is legal in a specific state, doesn't mean you can't be arrested for it by a federal agent. The ATF is simply stating that you cannot own a gun and be in violation of the law.
State's rights TRUMP any and all ******** Federal Laws unless they were part of the Constitution, this is not so it is ********.