The commerce clause of the Constitution is what allows Congress to regulate interstate commerce "among the serveral states." It's also the bit in the Constitution that Congress and the courts have abused over the years to allow Congress to meddle in things that they don't have Constitutional powers to meddle in. They come up with the wildest excuse relating their laws to interstate commerce.
Justice Thomas believes in strict adherence to the commerce clause, i.e. it doesn't apply to anything other than commerce between the states.
Mark