Dear Boston - you ask a good questions, but one that has several layers of possible answers. As has been shown in the last 20 years, the short answer is that the federal government is limited by the constitution. However, in reality, it finds a way to regulate anything that it wants, absent a challenge. That is my ode to the rhetoric aspect o your question. Regarding the specifics of your question, principles of interstate commerce, safety, and other issues provide the feds with vast powers. More specific facts are needed to address a specific issue.