Each site can have 4 custom close reasons, as Thomas explained, and on Unix & Linux, we have added a reason for cross posting. So the strange thing isn't that AU doesn't have the reason, but that U&L does.
For reasons that have never been clear to me, the U&L community is particularly sensitive to cross posts. So much so, that we implemented a ...
Do nothing. As long as it seems like a suitable answer we leave it alone.
This said, if the post is spammy, not related, or similar then we address it. Otherwise, we leave the questions be - no need to touch something that's not really 'broken'.