Absolutely none of the prisoners released by Bibi has ever killed an American and they pose no threat to the United States.
Karzai may believe that his best chance of survival is to make peace with his enemies. Bibi might feel the same (the US is rapidly losing military strength and moving to isolationism, and left to its own devices it isn’t clear what Israel’s long term prospects are). Releasing people who for the most part were young when captured, and are now middle aged, is a low risk way of trying to open a door.