I do believe that this is the silent agreement between all presidents- that no one will ever go after a previous for the crimes that they committed... she was the wife of a sitting president. if he were to break that commitment... well... we would have a lot of presidents in jail due to a cascading effect, likely starting with Trump. I have no evidence of this, but considering the nature of the crimes they commit and no one is ever held accountable, this must be the case. He didn't realize this until his conversation with Obama on the day he took office.