The default derives from the site's origins as a place for YC alumni to exchange links. HN is in effect like an alumni magazine that has gotten a broader readership.
Rather than a default editorial action, why not simply ask the community to put the YC info in the title?
Like I said, it changes the interest for me. I'm less likely to read marketing. I could be in the minority even in this case as a happy Airbnb user.
The YC alums are the editors, that's why some things end up dead and others don't
a blend of advertising and ingratiation
HN is YC marketing, it didn't really start out as an alumni magazine either, but once they realized that they could use it to, you know, sell ...