What stopped them in the first place? It's not like Presidents haven't walked a fine line in the past, with the knowledge that they can't actually be prosecuted. They have to have a certain level of immunity, much like cops, because operating outside of civilian law is a requirement for them to be able to do their jobs. They can be impeached and be protected by a divided senate. That's the "court" they have. Other than Nixon(and now Trump), charging a President with crimes when they become a citizen, has never really been entertained, and they pardoned him before the ball even got rolling.
I really don't see the big change here, other than just making the unofficial rules for President, official.