Whether or not it's legal in the state or country is irrelevant. The NFL is a corporation. A billion dollar corporation at that. If it's against their rules to smoke pot, then you shouldn't do it or you will be punished accordingly....
Exactly. Whether you think or don't think pot should be legal has nothing at all with this. If you go to work for a company that has a rule "no facial hair", and then grow a mustache you can't very well complain when you get fired, and saying "there is no law against mustaches" is not going to change that.
The NFL has a rule against Pot use. Lavon obviously knows that because this was his second offense, not his first one. So... he knew it was against the rules. He knew what would happen if he was caught. He chose to do it anyway, and got caught. There isn't much more to say. Let's hope for his sake that he doesn't get cut, that he learns from this, and that there isn't a third offense.