Here's what the Manafort indictment comes down to: It's not that he broke the law by repping for parties in foreign countries. It's not illegal to do that. And he wasn't acting as an agent of some sort of hostile power such that issues like treason would come into play. His business activities were legal, but he was supposed to fill out paperwork to register as a foreign agent. He allegedly didn't do that, so he was apparently in violation of the law.
But usually when the Justice Department finds someone to be guilty of this particular violation, they don't prosecute. They just tell them to get it taken care of.