Why do they steal his food?

They're not stealing it, they're helping him out. They know that he'll never be able to eat as much food as is on his plate.

There was a rule that if you don't finish all the food on your plate, you "spend a night in the box." (Part of the rules at the beginning.) He was so exhausted after his previous punishment that he had little appetite or energy to eat, but the staff loaded up his plate with more than the usual amount. His buddies all took a little so that his plate would be empty. Showing support and compassion, and gratitude for what Luke gave the guys.