There is a big difference between handing out light punishments and actively colluding. In the case of Cohen, his light punishment is because they were never able to find the smoking-gun evidence they needed to put him away properly on criminal charges despite a massive effort to do so so they settled for what they could get. It's not enough to be guilty (hi OJ), and Cohen was guilty as sin I don't doubt it.

If the punishments are smaller than the rewards from breaking the rules, then they have no effect on stopping the behavior. They are functionally a small cost of doing business.