Suppose a "Linux" question gets moved here from Stack Overflow. The question is simple, quickly answered, and accepted; but the question really doesn't fit ServerFault, it just doesn't fit Stack Overflow (it would be better on e.g. Unix & Linux). Should it still be flagged for migration, or just left alone?
If it's got an accepted answer against it, we prefer to leave it. Shuffling answered questions is kinda pointless.
However, there's no reason you can't answer it and migrate it...
I've always been a fan of having a kind of "professional courtesy" between the Stack Exchange sites. All of the stack exchange sites know which site a question should be migrated to (reasonably well, at least).
If Stack Exchange, Super User, or whichever migrates a question to us, I think we should do our best to answer the question or close it. It's at least a borderline decision if they sent it to us in the first place, and if we're considering sending it back, I'd say our responsibility to the person asking (and to the internet at large) is to do our best to answer it.
When in doubt, err on the side of punishing the innocent the least. In this case, that's the person who asked the question in the first place.