I agree with Blizzard, this imbalance isn't about one side's being more powered than the other. Cops are dependent on criminals for their gameplay. So all they can do is stopping criminals from doing something. When criminals do not do anything or when a cop fails to stop them on their own, criminal side looks more attractive. For example I do play as a cop only when I have a companion with me. Because playing as a cop when you are alone is not amusing at all.
This imbalance might be due to the players who do not have a companion/friend to play with them. So they basically choose to play as a criminal.