The demon jumped Sam and knocked him out. He dragged him over to an empty room and locked him in. This went against everything the demon believed in. When you see a Winchester, you try to kill them. OK, that never seemed to be very successful, but it made sense. This time it would be so easy. He had one all alone and totally helpless. But, for some unknown reason, Crowley had said hands off. It was the same rule Lucifer had. At least Lucifer's reasoning made sense. But not killing a Winchester because it would annoy an angel? Stupid.