I once saw a talk given by a lawyer on exactly this topic. It was a long time ago, unfortunately I won't be able to find it. Anyway, the takeaway is that there are plenty of Federal laws that are written in such a way that there is incredible room for interpretation by prosecutors. Vagueness and overbroad language to the point that indeed they can come up with some kind of crime pretty much any time they want to.

On the other hand, that kind of thing would not only be enough to bring a case. They use that kind of power to enhance their case against people they know are real criminals. Of course, the more the Justice Department becomes captured by bad actors, the less this applies.