That's not what I've heard.
You'll have to trust me; it is.
But maybe the reason more pilots don't do it is just that they have to give up a week of their own time to take the training
Yes, time and money are big factors. Also, the lack of "real" threats right now and the convenience of the Known Crewmember checkpoint makes many pilots not even consider the program. Known Crewmember (KCM) is a program that allows airline crewmembers (pilots and flight attendents) to go to work without having to go through TSA security screening. We show our IDs to the TSA agent who checks us against the database and if it all matches we are allowed to bypass security. We are still subject to random screening and other rules, but it is much faster than trying to go through as a FFDO.
Pilots are cheap. That is a given. So, few pilots want to volunteer a week of their time to go to Artesia to increase their responsibilities at work without a comparative increase in pay. Some people go just to get the quality training. Some pilots became FFDOs to bypass the TSA screening hassle, but KCM makes it easier now, so many FFDOs have dropped out of the program.
FFDOs aren't cops. They are not law enforcement officers at all and they do not qualify for the law enforcement concealed carry benefit that retired LEOs get. They are strictly deputized federal officers with very limited powers and jurisdiction. Big heads, but small jurisdictions.
Being deputized allows FedGov to offer some relief from liability, and more importantly, removes the airlines' liability for a FFDO's actions.