First, thank you for your informed take on this. Always nice to hear from someone with real knowledge on a subject.
Henderson's marijuana use predates his Crohn's diagnosis so we know his history is recreational use. He's also been in the NFL's substance abuse program for some time because a 4 game suspension is the penalty for a 3rd violation. There's no excuse for not knowing and following the rules in this case. This is on him and I have to think his NFL career is pretty much over.
Whether or not you agree with the NFL marijuana rules (and I don't), they're the rules. If you want to play, you have to follow them - or at least not get caught breaking them.