A friend of mine sent me this article/ad about the benefits of karate for children with autism. I've seen a number of these sorts of articles about how a particular activity is beneficial for children with autism and the conclusion I'm drawing is that the benefit is less about the particular activity and more about the opportunity to practice social skills in a welcoming environment.
The key feature is interest. A child with autism has to be interested in the activity in order for it to be worth the effort of the added social challenge. If they're hockey fans, then get them involved in local teams. If they like computer games, get them into the local gaming community. I could see it being difficult at first. We're all uncomfortable in new situations and it takes us all awhile to figure out the new rules. For a child with autism, this period would be even longer, but once they do find their social footing, they'll discover something amazing: other people who are equally enthusiastic about something the child cares about.
It's not socially inappropriate to talk about karate movies, karate techniques and endlessly repeat and perfect maneuvers in a karate school. Such a conversation would likely bore the heck out someone in the grocery store but the people in the dojo are karate enthusiasts. They like talking about all the finer details. Ditto any other particular obsession.
Any group which gathers for an activity is interested in that activity and will be happy to discuss it at a length which would discourage a random stranger.
Granted there's still some social refinement to be learned, such as turn-taking rather than monologuing or lecturing. And learning graciously rather than assuming a position as a self-taught expert. But those are easier without the added pressure of having to deal with monotonous small talk or uninteresting conversation topics. So I can definitely see how specialty classes could help a child with autism to develop their social skills.