Comment 42 for bug 861171

It's a toss-up: the current behavior is mysterious, but the proposed behavior would be vexatious. Ubuntu would be telling you that the other accounts need to log out, but refusing to offer to do that.

I don't feel strongly about whether it should require admin-only authentication or any-user authentication. The important thing is to have a confirmation step making it clear that forcing logouts of other users is Serious Business.