Legal and criminal responsibility for actions while governing leads to a situation commonly seen in a number of republics, most prominently ancient Rome, which the creators of the American constitution desperately wanted to avoid. While in office, you're safe to some degree with the powers of the office. Once out of office, you're an immediate legal target and all the lawsuits get filed. Solution? Don't leave office. Avoiding that is the basic logic behind immunity in political office; stable transfer of power breaks down when you know your successor is going to imprison you.