Another example is MPAA movie ratings. Since they decide which ages are appropriate for the movies that they license local theaters to play, theaters prohibit kids from sneaking into R-rated movies, even though it isn't against R law.
It's a shame self-regulation doesn't occur more often.
The MPAA rating system might look like a success from the outside but I think a lot of people familiar with it would disagree. It is wildly inconsistent and the rules that it does follow are mostly arbitrary (one "fuck" is ok, but two isn't) . The process of getting a film rated is not very transparent and getting a film re-rated can be prohibitively expensive for low budget films.
On the other hand, government rating systems can get hilarious.
Here in Finland, VHS and DVD distribution of program materials were rated for audiences (does it suit children or not, etc) by a government agency. They wanted to raise money for the government. So, even if a program was rated for VHS, distributors couldn't use the same rating for DVD distribution; they had to pay another fee to get the DVD rating. Without government inspection, movies had to be distributed with K-18 (X-rated) label.
Eventually, one of the distributors got fed up when they were told to pay yet again for the new rating of "Little House on the Prairie", the nostalgic 1970's family series classic. The distributor simply said no, and decided to sell Little House as an X-rated DVD release. Supermarkets had to verify that buyers had photo ID and were over 18 years old. Parents were told it's forbidden to show the series to their children (it had been on air in prime time in 1970's and many remembered it fondly).
Everyone started to laugh at the government rating agency, and they gave up and changed the regulation.
I met someone who worked for the British equivalent, the BBFC.
She said every release had to be submitted for classification, since they often contained different material -- a slightly different edit of the main film, or "extra features" which occasionally needed changing, such as where the director's interview "The Making of Fluffy Bunny Adventures III" had him cursing about something.
Yes, that's true. Here the good thing is that the regulator isn't really counting the swearwords in a film one by one so that a director's interview of Little House would really change the rating. Or at least that was common enough perception among the public.
Also, there's sometimes a long delay in providing a DVD / Bluray version of a film that had previously been released on VHS, and it's possible that the same film now gets a lower age rating.
It's a shame self-regulation doesn't occur more often.