Because people will post mirrors/copies and YC's very pragmatic in-house legal team has likely decided that those people bear the legal responsibility of doing that, or at least that YC's exposure is limited under certain circumstances.

That would likely mean that they will give out the poster's email/IP given a credible threat of a lawsuit.

So maybe don't do that if you live in a country where copyright is actively enforced.

Newspapers/magazines generally don't go after copyright infringements on an article level, I think, but who knows what might happen in the current economic downturn.