If done against the wishes of the owner of the site, yes, I would consider that unethical. Thankfully, Google respects robots.txt and noindex.
Especially since they're not moderating things or anything.
Agreed. However, it's probably covered by their terms of service.
Same thing with the recent reddit kerfuffle. I'd have much preferred a Usenet 2.0 instead of centralizing global communications in the hands of a handful of private companies with associated user-hostile incentive structures.