why did the corporal stop picking on paul and his friends

Tjaden refuses to salute Himmelstoss and basically tells him where to go. Tjaden is not severely reprimanded because strong experienced soldiers, like Tjaden, are scarce. The front is no longer a training ground and Tjaden is worth much more to the army well rested rather than punished. Himmelstoss's superiors reprimand him for picking on the young men.