Actually it would be a really bad precedent to prosecute a president after he leaves office. The Democrats will hold power again sometime and if the precedent has been set then they will, as a matter of course, prosecute former Republican presidents for whatever they can think up and will put them in prison.That would be the end of the Republic, should it still exist, because the people who would then seek the presidency would be all dictatorially minded. Total power would have to be secured along with lifetime tenure in order to avoid prison.
If a party loses power their administration is prosecuted and jailed and/or executed.
This provides a huge incentive to not have free and fair elections, to not give up power when elections are lost, to suspend or rewrite Constitutions, to declare oneself “Dictator for Life”, etc, etc.
Criminalizing policy differences as treason is not a smart move. And arming rebels in Libya wasn’t even much of a policy difference. Should McCain be tried for treason on the same grounds?