It depends. If you got them because you thought you were going to eat them at the restaurant, and then you take them home instead of letting the restaurant throw them way, then the restaurant doesn't lose anything. If you put more food on your plate than you would have with the knowledge that you can just pocket and take home whatever you don't eat, then you're stealing from the restaurant. They price their buffets with the expectation that you'll only be taking as much as you can eat while in the restaurant. Either way, though, it's minor. It's just slightly wrong, IMO. Besides, the restaurant probably anticipates that a few people will break the rules and take some home.