It happens now. This is rarely prosecuted at all, I think 7 times since 1938. And the ONLY reason it is now is Mueller is trying to squeeze Trump. He would have dropped this if Trump wasn't in the picture like he WILL do for Podesta, GUARANTEED!.....
It's rarely prosecuted in large part because usually the way that violations are handled is by contacting the person doing the lobbying and asking them to submit the disclosure. They still take it seriously. But Manafort has exhibited a pretty significant degree of duplicity with the federal investigation, so they threw the book at him.
I think you're wrong that Trump is the only target here. He's certainly in the mix, but Manafort is a big fish by himself. In fact, the first time a FISA warrant was issued for Manafort, it was before he was campaign manager for Trump, and most of these indictments date from that period. So, the investigation had started before Trump was in the picture.
It's true that part of this is showing to Trump and Trump's people that they need to take the investigation seriously. And that's a completely legitimate thing for Mueller to do.