Pirate and Executioner "leave town" after achieving their wincons. In effect, they are dealt an Unstoppable attack at the end of the night following, flavored as "They left the town victoriously."
Amnesiac specifically prevents other factions from winning so that it's able to remember a role before the end of the game and could gambit with a Godfather and remember SK to steal a win, for instance.
Witch wins ties in endgames against solo Town, as do other "True Neutral Evil" roles.
The goal here is to standardize the interaction between roles that can win before the end of the game and said endgames. By simply having them leave town it makes kingmakers less likely and also prevents that player's vote from swinging for or against particular teams.
Amnesiac has abysmal winrates. While the art of remembering at the right time is certainly part of the role, why should an Amnesiac be massacred for not remembering a role before the game ended? Instead, extend the game until Amnesiac remembers a role so it could choose to win with the other player or go against them. actually this solves more problems with kingmaking so you know what maybe don't do this one
Witch has bad winrates. Fact. Solve that by making it break ties in endgames in its favor.