Community user shouldn't always be online?
Actually, this whole "Community user" is a nice example of Technical Workarounds/Implementation seeping through the front-end.
I'm guessing the sites need a community user to assume reputation for certain votes, to close questions or wikify them, in short wherever an ID is needed.
The decision to implement this as Yet Another User was probably taken because it was the easiest, even though it might confuse users. That's why the probably added following explanation in the community user profile field:
I'm not really a fan of these kind of hacks, but imho they should be the lowest points on the "to fix" list, it's just an esthetic's issue.