The tenth amendment: "The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the People."
For about a century now, the US Congress has used the Commerce clause in the Constitution as an excuse to pass any law it wants, and only rarely gets its hand slapped by the Supreme Court. Lately some of the states, especially those with more conservative constituencies, have been staking out areas they feel the federal government has no business in.