That's not much of choice when you don't know you have an option and someone suggest "Post this on Ask Ubuntu instead", is it (and this is what I've seen many times in regards to Ask Different, the Apple site)? You do what that friendly person — or web site feature — tells you to do, since you didn't even know you could. You don't realize that it's merely advertisement for another site.
Many issues users with Ubuntu face aren't actually specific to that distribution. Posting these questions here might actually increase their visibility and usefulness for other users.
Ask Ubuntu isn't in beta anymore, which means it's strong enough to grow on its own by attracting new users properly instead of blatantly stealing them from other sites on the network, and doesn't need special considerations by what can be considered the "parent site".
The same applies for suggestions that Mac/Mac OS X related questions, which are on-topic on this site, "should" be migrated to Ask Different. Not necessary. Not going to happen.
This would, in the very long run, just hurt SuperUser.