It's because the German state has a special commission to decide what is harmful to young people. Which is funny: in a country so obsessed with personal freedoms and fighting fascism, nobody seems to have a real problem with the existence of an institution that encourages (or rather, forces) censorship.
Everyone with a German IP is buggered. Good thing Steam doesn't retroactively limit access to your games.
Note: VPN is a bit dodgy as far as Steam is concerned, I recall the TOS specifically forbid this kind of circumvention. If you do use one, try to make sure it's set to a neighbouring country without fucked up regulations.
If Steam asks, tell them you had to shell out and travel to the country next door, to exercise your customer rights.