People expect something that was not intentionally sent by their friends to be actively intercepted. While making the technical distinction may sometimes be tricky, the emotional distinction is fairly straight forward.
You're right, it's an emotional distinction. Let me illustrate. A man operates a gay rights message board that includes private messages. He tolerates alternate viewpoints, but after a particularly bad flamefest he uses a plugin to ban linking to a hate speech site that is especially contrary to the aims of the forum that he pays for out of his own pocket. It extends to PMs. He explains that if you want to trade such links you're welcome to go elsewhere.
But the aim of Microsoft's messenger is not to prevent intellectual property theft.. especially when the users are under the impression that it is to facilitate private communications..
> He tolerates alternate viewpoints, but after a particularly bad flamefest he uses a plugin to ban linking to a hate speech
So he doesn't actually tolerate alternate viewpoints, the problem is in the framing. If the discussion board was supposed to facilitate discussion of opposing viewpoints (like IM network is supposed to facilitate private messaging) then it's not reasonable to ban one side's resources.
