I suggest googling for "tsa theft"; if they're firing hundreds of people a year every year it isn't a negligible problem. But even if more is stolen by airline employees than by TSA agents, that isn't a defense of the TSA. We couldn't fly without the airlines; we flew for decades without the TSA.
As far as I know, the amount of theft due to TSA is negligible; virtually all of it is due to airline employees.