It's meant to stop the completely honest children that will see it and not play. Because some children know they should not play a horror genre game, as most parents know their children should not.
But mostly is is meant to be a disclaimer. This way if there are complaints from angry parents, it can be said "hey, we have a age limit and it is clearly posted". There is not a whole lot STS can do to enforce the restriction, but it is meant to cover them more than anything else. And my guess is if a parent complains, the account will be closed by STS with a "thank you for bringing this issue to our attention" letter.