This is different from SO and MSO. Is this a bug, or the desired behavior? If this is by design, why?
not just a little different from SO and the other sites, and it only happens in questions that have lots of answers (or are CW?). (Example SU question where comments are not hidden)
Edit: A relevant blog post from April 2009 says:
On questions with >30 answers, only
comments of +1 or higher will be shown
...but this does not seem to be the case exactly, at least no longer, because in your example there were slightly less than 30 answers.
But whatever the exact condition that triggers this mode, I personally find this little feature very well thought out (as I commented back then).
Edit2: It seems Shog9's answer has all the details