When someone says "this is a game, it should be fun" they are usually making an argument toward being able to play however they like without paying a social price for it from their teammates. They think its ok to troll pick or feed and nobody has a right to say anything about it.
League wants to be a "sport", which for arguments' sake we'll call an organized competitive game.
The "fun" in sports is in developing both individual and teamwork skills to be able to perform well in the game environment.
The fun in sports is performance.
People who come into the game to screw around and throw the game for their team aren't helping everyone have fun, they are undermining the experience for 9 other people.
It's not toxic to call someone out for playing like crap and refusing to get better. It is toxic to have someone trolling your team and being completely unable to even say anything to that person.