Uncocking such an action with a cartridge in the chamber would be asking for trouble. The firing pin would be resting on the primer and it would not take much force on the cocking piece to set the primer off. It would be equally ill advised in the case of the US Krag and 1903 Springfield rifles, both of which were equipped with grasping knobs on the cocking piece.

I think the knob is there for the purpose of allowing the action to be cocked in the event of a misfire, without having to open the bolt, obviating the danger of a hang fire going off with the action unlocked.