Let's get real here. The vast majority of people who work for the Commissioners' Offices and Rule Committees for the major sports don't, realistically, care all that much about "the integrity of the game." They're business people. What they care about is perception and image. Coffee is going to help your football performance far more than pot, but coffee isn't illegal. There's a reason why the list of banned substances for any sport coincide so closely with the list of illegal substances in the United States. Don't let your players get away with illegal activity, you make them look like better citizens, and you improve your sport's image.
The treatment of PEDs and other illegal drugs in baseball for decades highlights this point beautifully. When the public didn't know about it, nobody gave a damn whether players wanted to use. Players obviously knew. Bud isn't an idiot. He had to know. Managers knew, owners knew. But as long as fans didn't know, it didn't hurt the public perception of the game. So let's get real. MLB isn't legislating steroids, HGH, etc out of the game because they want to promote the integrity of the sport. They're doing it because PEDs became a black eye, and in order to restore their reputation as the "wholesome sport" they had to aggressively handle the situation. Same is basically true of the NFL. Their big steroid era happened earlier, but it was overlooked for some time as well. Until it started to look bad.