Your are partially correct about the airlines not being able to trump just any boarding pass with SSSS on them. However, the airlines CAN exempt certain ones such as infants & young children, military on official travel orders (reservists, guardsmen & active duty) as well as active duty military personnel, and they can exempt those passengers who were not previously a selectee but were selected for the current flight only because their previous flight was cancelled. There are, contrary to popular belief, common sense provisions in the current policy. The problem is one of human dynamics: it is easier to go with the path of least resistance than to assume responsibility for a decision.