You seem to be implying this is the only thing they have done, which is odd.

I think they should go after big fish of course. But there's also value in going after the littler fish. You don't want people thinking they can get away with insider trading if they just keep their crimes under the big-fish threshold. After all, there are a lot more little fish out there, and most serious criminals have a long pattern of escalation. It's valuable to keep people off the path entirely.