Naszrador: But I think sales were some reason for the censorship. If they need a 16 or 18 rating they have less customers to sell it to.
Sarisio: Problem with this is that you need to be able to prove that buyer is of eligible age, which is very troublesome for digital distributors. It is extremely troublesome, it is one big swamp, which also involves compromises of personal information and such, so it is easier just to censor game or not bother with distributing it at all.
If to ask me, the whole ratings system is nonsense. Pre-18 people nowadays have very easy access to pr0n and violence stuff. Restrictions (and censorship) bring nothing but headache for legit distributors/users/players.
Law restrictions for minors aren't so much nonsense than simply innefective. 18+ Retail games/videos may be hard to get, but some download/stream where you just get asked how old you are? Useless and just there to please the law.
And Steam already has a lot of 18+games, mainly due to violence, so it's not like there is a problem with trying to sell adult content digital. It's rather easy and simple for them.