I think a major pint being ignored is that even if damage is done unintentionally, a kidnapper robot can put another team out of a competition with severe damage.
I am NOT saying that all of these bots have evil intent, but their actions, by lifting another robot, increase the chances of damamging another robot.
I dont think anyone can refute that fact: By lifting a robot you are making a bet that the robot will survive unmarred. It is a gamble, and using another team's robot as a playing chip makes it a serious one.
As for comparing these robots to ball grabbers or goal mover being threatening on the field

...
You can always disengage when losiing a pushing war.
You can always try and outmanuver other bots if you are a ball grabber

and get balls into another goal.
But when your robot is picked up, there is nothing you can do, and since colliding with them with the intent of jarring them loose is illegal (damage) they are protected by the ruels they exploit in order to pick up other robots.
A clear danger? No (did i say that, anyway?) But it does represent an increased risk. I don't think anyone can argue with that!
--Ben Mitchell