I strongly dislike how much power is gained by the mobility of having access to a grappling hook. Giving this to the traitors would seriously boost the lethality of the traitor team. It is a trade off, since that would make it traitorous to use a grappling hook, but that essentially pushes the meta more towards deathmatch by increasing the power level at the cost of reduced stealthiness. For anyone who thinks that bhopping traitors are annoying to play against, the grappling hook requires less skill to use, allows for faster movement and acceleration, negates fall damage, and allows for movement in any direction. I think this would be extremely boring to give to traitors.
You make an extremely good point about common sense though. I was going to -1 this, but I'll stay neutral for now because I don't have a better solution to the common sense problem other than proposing that common sense kills just ignore the potential for grappling hooks, which I think would be antithetical to the nature of common sense.