The reasoning is simple.
New players are getting used to the orientation of the keybinds, what to do in game, especially since this game has a high learning curve for particularly new players.
Getting killed out of nowhere by an antag, especially the things antags can do to them can be particularly frustrating to new players. As a leniency for them to get used to the game, they should be able to be opted out from antag objective until they are ready to be killed.