I think the overall point here is that the ability for a message addressed to one person to end up on someone else's screen carries some rather unfortunate implications for the internals of the service. Compare with IRC private messages, email, heck even XMPP.
I know XMPP does because I use that particular function at work daily, and IRC supports person to person PMs.
What I meant though is that the fact that whatever kind of shenanigans they're doing internally has a failure condition that can misroute messages is a bit scary.
