Often the people who submit links do put the YC info in titles. But it wouldn't work to make it a rule that one had to, because a lot of submitters wouldn't know what batch a given startup was in.
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 ...