If they don't level you down for them then they are going to have to ban you from them. They can't have level 40's wandering through a level 15 zone event one-shotting everything in sight. This way your high level character can PARTICIPATE in the events rather than ruining them for everybody. If you don't want to get your character auto-nerfed for the event then stay with events designed for you but, if you've got a friend that's new to the game, this will allow you to play together without having to roll up a special character just for that purpose. The "lockstep level or lose your friends" rule is finally dying.
City of Heroes actually has 'level neutral' enemies for zone events but zone events there are more of a novelty - just fun little things that happen from time to time and give everyone a chance pile in on a big furball of super powers. In GW2 the zone events are going to be a way of life so they will need to be a lot more careful about balance.