Corrupt politicians can simply ignore the law. If they didn’t ignore it, they wouldn’t be very corrupt.
This is not true in France. Politicians that have proven fraud are arrested and charged. In France we have Sarkozy, Cahuzac, Fillon that were all charged with crimes.
They were president, minister and presidential candidate respectively. I’d be surprised if it was different in the USA. I’m seeing that trump is also being charged, the system seems to be working.