It's not just silently installed by Steam, or something, they have to explicitly confirm they accept it? I don't play this game, I am curious if players are unaware or actively stupid.
Riot's games can only be played with their own launcher, so no steam. They give a big message about "now installing vanguard, our anticheat," then inform you to reboot to finish the install, since vanguard is the only ring 0 anticheat that puts itself into the kernel start up, always running.