With limited resources, the locals are usually happy to let the feds, especially the BATFE, invest the time and effort. The feds, in turn, are happy to generate the publicity for fearless leader, whoever that may be at the time. I suspect that
@Ohen Cepel has this right. After all, it is against the law to assault, batter or kill someone regardless of the tool used.
There is a method to this madness, however. More laws generate more plea fodder that help compel suspects to take deals. If one prosecutor fails to obtain a conviction, it is sometimes possible for her counterpart to prosecute under the law of a different jurisdiction or when different elements must be proven.
OP, the machine gun ban occurred with the 1934 National Firearms Act or NFA.