I play WAY better when I'm high. I think you're barking up the wrong tree. You should not play when your mental state, for whatever reason, is impeding on your ability to play.
Proposing that specific measures be instated for this, however, would undoubtedly create a slipping slope. What if you are in a region where it is legal and, like me, it allows you to perform better by counteracting the crippling anxiety that would negatively impact my performance otherwise?
What about Prozac? What about anti anxiolytics in general? Or antidepressants? Anti psychotics? Caffeine? All of these substances have known and measurable impacts on cognitive functions; Should we ban all people with anxiety disorders?
Better still, what if you are sleepy? Hungry?
I could be convinced to support IBEAPANDAA's position, in that telling people you are under the influence can affect team morale. But to prevent people from playing? That's just selfish and misinformed.
Speaking of misinformed, I don't know on what grounds you would propose to sue Riot for this, but you would lose the case and a lot of money in lawyer fees.