So Riot's excuse to a troll system is "it could be abused"...
Basically, some poor soul of a toxic 4-man premade that want every role they want or they'll all troll and feed, can get kicked and punished for not complying. But why not add it?
Seriously, at the moment, at lower elos, probably at least 1/4 of games have trolls. Feeders, afk'ers, people that just plain won't follow pick order and will go double mid because they didn't want support. But with this, sure, some people would be affected, but I doubt those people that are being trolled by premades haven't been trolled before by trolls, or heck, premade trolls. There's almost no difference, the random guy has to dodge or face a whole game of toxicity, and overall just no fun. So with this system, we could probably cut trolling in half or maybe more. You could say "the ends don't justify the means" but it really does when it comes to the fact that the few people being punished are going to be punished by trolls and premades anyway. There's almost no way to get away from it and implementing it only to have it stop half of the trolls is more than enough reason to implement it.
The only other reason I see is non-meta picks being kicked. But lets be honest, people troll and feed because of off-meta picks anyway so the same problem would be presented. And this wouldn't be near as much of a problem if Riot would just let us report people out of the end-game screen.
Edit: ... I feel like since Luck5962 trollposted everyone is now downvoting me. Everyone knows what I fucking meant. ;-; Its like a mini circle jerk.