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The claim isn't limited to the scope of the site.
In addition, certain things that one would call creepy wouldn't be more legal if done in public. It would just be stupid and a lot easier to catch them.
Yes, it's public but you're taking the definition to an extreme. They're called private messages for a reason. The only person who could read them that isn't part of the conversation is the server owner and I can assure you that he doesn't want to go around reading people's private...