There is also no "rule" saying that people can't bloodbend without a full moon. We just hadn't seen it before. Why did Azula always bend blue fire instead of red? Why can't all Earthbenders also metalbend? Most Waterbender probably can't bloodbend without a full moon because they would have absolutely no reason to try. Yakone and Sons practiced; they got started at a very young age. I bet if Katara really wanted to, she could bloodbend without a full moon.