If it was any other player on the roster they would have been benched with the type of horrendous mistakes he made.
Furthermore, the battle went to the last day of fall camp. It's not like this was a no brainer decision. You have a guy on the roster right now that has lots of P5 experience. Why not give him a shot? I seriously couldn't be much worse.
This is the type of stuff that loses a team. You can't allow a QB to make these types of mistakes without repercussion.