Hackernews started out as a way for YC to talk to their would-be founders (it was called 'start-up news' originally) and the rest of the postings are the noise and those are the signal. If you are unaware of the relationship between YC and HN you could be forgiven but after more than a decade here I don't think that excuse holds.
Marking job ads more explicitly won't change anything, it is just a way to pressure YC into making some change to satisfy nit pickers and there is far more important stuff to do, and is as far as I'm concerned off-topic given that the question was 'why can't we comment on these postings' and that question has been answered afaic.
Furthermore: the who is hiring threads are there as well for everybody else as well, which given the audience and the fact that these compete directly with YC funded companies should be proof positive that no malicious intent was ever present. If it were then those threads would have never happened.