Because, as you said, many are not really PvP’rs, they are just there for the bonus, and when PvP happens they don’t know what to do.
I have a suspicion that the A:H balance would be a lot better if all the PvE’rs were removed, which is why I’ve always said we should remove any PvE incentives from the system and replace them with PvP incentives.
But there are Horde players that really want to, and can PvP. They are probably also not happy with the PvE’rs in WM.