I don't think that's how sharing someone else's post works. If I make a friends-only post, and you share it to your friends, only the intersection of our friends can see it. I agree this is maybe not as clear as it could be on the UI, but I believe that's how it should work. If you find otherwise, would you mind filing a bug at www.facebook.com/whitehat?
Another FB engineer here - I actually work on the system that caused this false positive. You're right that if we were actually trying to stop constructive discussion from happening, that would be bad. We're definitely not trying to do that - in fact, our goal is the exact opposite.
Similarly, if we let through blatantly malicious or spammy comments, I think that would be bad as well.
While I wish I could say we were perfect at stopping spam, the reality is that no spam classification system is perfect. Sometimes we err too far on the conservative side, and spam gets through. Sometimes we err too far on the aggressive side, and good content is incorrectly blocked. That's what happened here - it turned out one of our spam classifiers was a little too aggressive, and we've turned it off.