Mustrum_Ridcully
Legend
I tend to think they are calling it once per encounter abilities because that is the design philosophy behind it. The actual implementation (mostly for easier "book-keeping") of it might be more like "use and recharge", but that's not what the design philosophy is about. The implementation might (theoretically) allow you to recharge during a combat, but that's only because it is so hard to put a generic description for an encounter that will suite all possible players and Dungeon Masters.