Yeah I understand, I was used to a bar being filled after cap from some other games too. Maybe they decided they want you to gain absolutely nothing after cap, and not give you a level ready for when it's raised.
Anyway, some people just keep the leftover story quests for their second class, I personally think it's not such a waste even doing them at 50, if you enjoy the story. After all xp is not hard to come by in this game.