I actually support this move. Way too many people leaving WZs mid-fight. Not only does it inconvenience their teammates, but it's not fair to the poor sod who gets dropped into the middle of the warzone to take their place.
This was standard practice in WoW. You were not allowed to queue for both BGs and dungeons at the same time.
This game isn't W0W and this change will have a VERY minimal impact on people leaving WZ's.