It's the default, which means the question is not when to add it, but when not to, and it's too much work to decide not to in a few cases where it might seem wrong for some subtle reason.
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 ...