The Commerce Clause is the one way that the Feds can regulate behavior. The reason you have things called federal crimes are because (1) they pertain to something which the federal courts of "original jurisdiciton" which means they CAN'T be heard at the State level (copyright, etc). and (2) legislation has been enacted to deal withi it pursuant to the commerce clause.
In short, anytime a behavior involves crossing state lines, the Commerce Clause is the trump card by which the goverment can regulate. The big controversy is that the COmmerce clause has been read SOOOOO broadly, and according to many scholars, way more broadly than it was intended. Like with federal drug crimes - how are they able to catch a drug dealer who only deals in-state, and assuming he doesn't import the drugs himself (which would also trigger commerce clause power)? By saying that the phone calls he made to his drug cohort in another state cross state lines! It's really insane, actually, and a classic example about how the constitution has been twisted and extended to keep up with modern times. Yeah, sure, in the 1800s when there really wasn't much "globalization" within America, States could satisfactorily govern things. But now that we buy milk inour local supermarket from Kansas, our Oranges from California, and the like, we are much more integrated as a country and there has to be a federal ability to regulate. That's one side of the argument. Of course, like above, the other side is that the Feds are robbing the states of their rights.
See, after all, our country is based on Federalism, which has by definition to layers of competing rights - the rights of the federal government to govern, and then the rights of each individual state to do so.
It is a crappy course, although it appears to be the most interesting, I'm sure, to you all. The reason it sucks is that you say how disingenuous and how politicized our "laws" are, with judges writing diatribes that would make Dickens proud, but that really stand on nothing more than their view of how government should progress. The trick of judges is to disguise their own beliefs in supposed rock-solid logic...Read Gore v. Bush for a shining exampe.