There is a sports stackexchange. Should we redirect questions about 'rules' and such to that stackexchange... ? Or do we accept that, while not martial-arts-questions, they are martial-arts-related and acceptable?
Rules questions regarding the user's competition in MMA/BJJ? Okay by me; they relate directly to their participation in the combat sport, which falls under Martial Arts.
Rules for people to understand UFC/Strikeforce/K1/etc. when they're watching should be outside the scope of this board. Unless we suddenly have an influx of known fighters with records in these organizations who can answer these questions objectively or in a "good subjective" manner, it's not proper. Besides, if they want to know the rules, they can look up UFC Rules, Strikeforce Rules, or any of the others by using the same search engine they used to get here!
This should be part of the "What Questions Can I Ask Here?" FAQ: some point that differentiates between participating in an overall sport and rules clarification for those who want to watch UFC on Saturday.
I've always thought those questions to be perfectly acceptable here as they are part of particular martial arts, so I'm just echoing what @stslavik and @DavidHClements have already said. We might be able to offer insight or expertise given our site topic and scope, but I wouldn't doubt that someone on Sports could answer the question either.
The overlap is fine, and it seems rules questions for martial arts would be appropriate for either site. One important point though is do not cross-post on multiple sites. Pick one site and stick with it. If you are questioning which site to post to, then you should be rethinking what you want to ask and go from there.