I'm sure we've all seen questions asked on MSE by new users, who think that this is a place to ask questions about parenting (as one did just tonight, but was deleted very quickly) or programming or any other topic under the sun. I think (though I have no evidence) that this happens because people hear about "stackexchange.com" being a good place for getting answers to questions (which it is), but don't realize that we have a network of sites. Meta is probably the site that comes closest to being a central site of Stack Exchange, so they'll post their "blatantly off-topic" questions here.
I think one way to help fix this is to block users that don't have accounts on other sites from asking questions here. In any event, they shouldn't have anything to ask about (bugs, site policy, etc.) if they don't use the network at all.