I'm saying that when you restrict your applicant pool to TSA screeners, you're unlikely to get as good a set of candidates as if you had opened it to the general public or those with LE or military experience. TSA screeners aren't necessarily unqualified to be FAMs, but for the reasons Law Dawg mentioned, I'd be a lot more comfortable with FAMs who had more experience looking at people and situations, trying to find crime and criminals, and seeing which techniques work and which don't.

Most of the people I know believe it takes about five years for a cop to reach the journeyman level, where he's not so much learning and training as acting. That's highly generalized, but I think it's basically true. Cops get the benefit of many interpersonal encounters each day--FAMs might get a few a month, if that. Given the gravity of a situation where the FAM might have to intervene, and the few opportunities he will have for less-critical situations to "practice," it makes sense to recruit FAM candidates who already have some of that critical experience.