He couldn't have pulled back the lock, they couldn't simply have climbed over the sides of the stall in all of three seconds, because those weren't the rules of the game. Theirs was the intoxication of the hunter, his the terror of the prey. Once they had actually captured him the fun was over and the punishment more of a duty that had to be carried out. If he gave up too early there was a chance they would put more of their energy into the punishment instead of the hunt.