A player -- an employee -- could go home every night and drink three martinis or down half a case of beer and no one would really know. The next day his BAC would be fine.
If that same player -- employee -- went home and smoked a blunt or two, the THC would show up in his system. The employer would be within its rights to terminate him, yet the evening before he was no more impaired with the blunt than he was with the three martinis.
It's hypocrisy and cultural political correctness on the part of the NFL. The fan base is overwhelmingly white and Republican. Alcohol is okay, because that's what everyone did back when America was Great. But pot is evil, hippies and radicals smoke pot. It's when our society went to hell back in the '60s and '70s.
The fan base, not to mention the ownership, is overwhelmingly white and Republican. The players are overwhelmingly not.
If the NFL were to take pot off its list of banned substances there would be a cultural meltdown. It's okay for Dad and Uncle Ernie to go down to the VFW and fire down beers and get blind drunk. But pot? No, sir. Just one more sign of the cultural apocalypse. What affect would it have on the kids?