I promise you, a lot of NFL players smoke weed. I wouldn't be surprised if it was a majority. For most, it's not that difficult to stop when they know they need to and pass a test. Most of them don't suffer from the anxiety that Gregory clearly suffers from.
Different medications work differently for different people. I'm sure he's been prescribed NFL-legal 'scripts and perhaps they don't work for him. People can deny it all they want, but weed helps many people (myself included) relax a lot. I never told my doctor I had trouble sleeping (2-3 hours a night in high school) because I didn't want to take a sleeping pill, due to the side effects and the crap that's in them. When I got to college and started smoking, I never slept better. Which led to me feeling better in the morning. I was more productive throughout the day. It was great for me.
I don't feel bad for Gregory in the sense that his life is at risk. He'll never OD from weed, and he's very unlikely to move on to other things at this point. I feel for him because this should be a non-issue. Weed probably helps him in downtime. By suspending him and potentially ending his career, who is the NFL helping? They certainly aren't helping Gregory.
I liken this to my own situation in a lot of ways. If I were to catch a marijuana charge when I was in college, I could've lost financial aid that I desperately needed. Of course I wasn't being drug tested, but I smoked often. I wasn't hurting any one. I was pretty much only helping myself. Helping myself sleep, study, be more productive. Taking away financial aid (and, thus, basically taking away my education), would've been way worse for me than just continuing to smoke. It's a victimless crime until it's against the law; at that point, you have a lot of victims.