They didn't initially know, but they have essentially put two and two together -- they look like bones + contain Elto energy = Elto bones, essentially. They would certainly accept the bones if given them, but they also would object to the bones being taken. They're very complacent on the issue; on one hand they're useful for energy purposes, but on the other they don't need all of them. Since the coven are sort of godly in their eyes still, they'll let them do as they please with the bones within reason. They may get mad if, say, the coven just took a bag of the bones and starting smashing the shit out of them for instance.