Therein lies the issue. The "list of approved supplements" is hosted by a third party, Aegis Shield. All they do is verify ingrediant labels do not contain a banned substance. If they don't have it in their database the player can submit a copy of the ingredient list. The problem is that sometimes the products are tainted by unlisted ingredients.

This is from the NFLPA's Drug Policies and Resources page:

"The NFLPA does not approve supplements. The AegisShield app is available to help players check the list of ingredients on supplement labels against the NFL’s list of banned substances. This does not mean that taking a supplement will be safe as far as the NFL is concerned. Supplements can contain substances not listed on the labels, and the NFL holds you responsible for anything in your bodies, even if you take a substance accidentally. The final responsibility rests with you to ensure that you do not take a banned substance."

These players are not taking something they know is banned for any competitive advantage. They would have to load up on something to gain anything material and then they would know there would be no way to avoid getting caught. I've heard that virtually every case deals with unlisted ingredients even on products listed as safe. The only way to be totally assured would be to avoid supplements altogether and that's not going to happen.