Ok. Here lies the problem, then.

Suppose, hypothetically, that a rogue TSO decides to go on a personal crusade against drugs, and decides to conduct primary searches for drugs, even though it's against TSA policy to do so. From what you've said, it's absolutely possible for that TSO to get away with that violation of policy, because he can always cleverly cloak his actions within the discretion given TSOs. If he sees a bag that he suspects contains drugs (for whatever reason), he can always find a legitimate excuse to call for a bag check, which would result in finding the drugs. Voila; TSA's nuanced policy on "don't look for drugs, but report them if you find them" can be effectively neutralized.