BDOs are not tasked with observing their co-workers. They've had background checks and are presumed to be above reproach until proven otherwise by an outside agency.

Besides, misbehaving TSOs know they are above suspicion or accountability, so they aren't nervous or stressed.

Meanwhile, the bad guys can study and train. Micro-expressions? Botox the heck out of your face. Whistling/yawning? No-no's. Contempt of security? Keep in mind that means that praise for security is also suspicious, because it's what an unpredictable bad guy would do, knowing that the BDOs are expecting bad guys to openly display contempt - but savvy BDOs realize this and now focus on anyone who seems to enthusiastically support TSA.