And this is supposed to prove that the federal government actually has authority to define other crimes why?
You can't arrest and prosecute anybody for an act, if there is no law on the books against it.
So you're saying that it's legal for a man in Arkansas to kill a man in Texas because there's no law preventing it?
There is a law against it (laws don't necessarily "prevent" crime, by the way, only prescribe penalties for violating the laws)--murder is illegal in both Texas and Arkansas, according to state laws; there is no federal equivalent to state homicide law. --------------------------------------------------------------------------------