I could argue it either way. I, too, believe in the free flow of information. But...
If someone sends me a message in private, it's obvious they didn't want it broadcast. Otherwise, they would have just written it in a public post. My instinct is to respect that, and I've never repeated a PM.
Oh, there have been times when I wanted to, because I do get the occasional juicy bit of gossip. But if people think they can't trust you to keep private matters private, they'll stop talking to you. I place a
very high value on whatever trust others grant me.
There are exceptions to every rule, though. Threats and insults, for example, don't deserve privacy.
Bottom line: we don't need to amend the TOS. Only send PMs to those you trust, and if you want to be one of those trusted confidants, honor the sender's instructions when they say "this is between you and me".