HN offers a publicly available API. Obviously using it (and by extension creating tools and applications that use user and comment data) is within their terms of service, there have been many, many such posted here over the years. YC doesn't have a problem with them as long as they aren't commercial and don't violate YCombinator's brand. But even then I've seen a few projects get real close to the line without consequences.

I don't think you understand what a "non-exclusive license" is.