> The spirit behind the idea of "public figure" in defamation suits is that these things should not be allowed to become tools used by the powerful to protect themselves from the weak.

No, it's that enforcement of laws against defamation, as an act of government, should not intrude upon freedom of speech more than is essential to serve a compelling government interest, and that legitimate government interest is at its nadir when the issue involves restricting criticism on interests of public importance, which includes public figures (whether general public figures or limited purpose ones within the scope in which they are such figures.)