There must be some logic to this, but I've seen it myself and haven't managed to figure it out yet, either.
In my case, at least under 10.3, clicking the eject button does nothing at all (unless the network item is selected, in which case it deselects it)--it doesn't even change what's in the Finder window.
I do know I generally see it at work, where a network is actually present, but not at home, where there is none. Maybe that's got something to do with it. Or perhaps only when there's a Windows network present?