Because piracy is against the law in most countries and games of pegi 16 or 18 can be played by minors if allowed or supervised by their parents,.
We also can not have control over how old someone is when they play on our servers, how ever we can try and control piracy discussion and the sharing of piracy related items, hence why one is permitted and the other isn't.
Hope this gives you an answer.
Also this is the SA:MP section, not the general section, please do not make general wide topics here.