Immunity. TSA has to grab junk all day long (it's their mission statement and let's be honest, they really like it). So a law would have to be written to say it's not the act that is illegal, but the intent.
So how do you prove intent? In this case, it's a slam dunk from the statement I've read. But laws aren't written for one-offs and it wouldn't apply in this case anyway (after the fact). Write a law that says unnecessary screening is a crime and might scare screeners from looking into suspicious situations--which really is their job.
So if they outlaw junk grabbing, TSA can't do it and a terrorist would wrap det cord around his dick. (I'm half joking here but seriously you know someone would try)