As you said, there are laws. The price a dragon pays for having treasures is that they must be treasured. If a dragon is unprepared to destroy a kingdom over a single stolen cup then the cup is worthless and so is the dragon. Likewise, in accepting one's place as in the hoard one becomes a treasure to be defended and admired. It is not that the treasure commands the dragon, but that the treasure would be worthless if it had no power. 

"Master," said Ailee in a voice she hoped was more than a squeak. "What do you seek here?"