The NYC Commission on Human Rights has a complaint process documented at NYC.gov/HumanRights. If someone has evidence of an actual violation perhaps start there rather than posting unsubstantiated accusations and implying illegal behavior on the part of un-named employers and YC.
I'll also note that the person who did post about this perhaps left the company names out because they were being considerate, not wanting to invite trouble for people who could be entirely well meaning when they are just examples of a broader problem. So as long as you're arguing for assuming good intent, maybe you could try that yourself?
Whether YC would want to prevent illegal or unethical behavior is a question I can't answer. I would hope so. But I don't think it's reasonable to put the burden of enforcing various employment posting laws on YC/HN, especially when the laws are new and have jurisdiction questions and exceptions. My reading of the NYC law tells me job boards are not subject to any enforcement -- only the companies posting the job ads have to comply.
I don't object to a discussion about what responsibility YC and HN have in regards to allowing job posts that don't include salary ranges. Personally I would ignore job posts that don't voluntarily include that. I objected to the OP characterizing the job posts from YC companies as "breaking hiring laws." Whether the law was broken or not would require more facts than the OP gave. Maybe the OP didn't name the companies that supposedly "broke hiring laws" out of consideration, but in the original post it seems clear OP intended to call them out in comments but couldn't comment on the posts, not that the OP decided to take the considerate path of not naming them.
Even assuming the best of intent -- in fact, especially when we assume that -- YC's role as a guide to often-novice entrepreneurs means they should be helping out here. Both by educating them on changing hiring laws and by sending back inadequate job postings for reworking.
Regarding your last paragraph, I'll also note that whether "the company broke hiring laws" is a purely factual question, as is whether or not HN publishes job ads that break the law. A generous interpretation involves why and how they did it, but can't include denying the facts.