I'm really not surprised. I assume every website does this. On my Linux laptop, I run NoScript, and I've got Firefox configured to clear EVERYTHING when I close my browser. I don't run Flash or any other plugins. I haven't done much inspection, but the whole notion of "logging out" doesn't mean much to me. In the context of cookies, I just assume they clear the one that says "this guy is authenticated", to "not authenticated, but we know the last person that was here".