Weed is dangerous. And for the sake of all the players, the commish should ban anything that endangers their health...besides football, that is.
Seriously, a player can be prescribed THC by using the Rx drug Marinol, strictly for medical purposes, much like other such substances that are banned in the workplace, like opiate analgesics. In most cases, the metabolites of these Rx drugs are specifically identified on drug screening, and the patient is safe due to his prescription.
Now, I'm not saying that the team Doc should be writing blanket Rx's, but a bona fide diagnosis through appropriate subspecialists may warrant THC therapy, and in such a case the NFL should allow such treatment since the medical field does.
A similar conundrum exists with the use of low-dose amphetamine salts for the treatment of ADD or ADHD.
If your car is searched ('cause you look suspicious) and your bottle of Adderall is found in the glove box, there is not a problem as long as it it current, has your name on it, and the count is correct relative to the dosing schedule.
OTOH, if you have crushed a few of those pills and put them in a sandwich ziplock, your dumb*** is goin to the slammer.
Even if you have some lose pills, intact, no printing on them, just laying in the drink holder, you're probably gonna be hauled in on suspicion.
They have to stay in the bottle even if you have an Rx, or you might be diverting them; that would be a controlled substance uncontrolled.
So, weed in its raw form is still federally illegal, just like methamphetamine. In Rx pill form, on a controlled dosing schedule and monitored by a certified professional, it's not.