Why are comments disabled on job postings?
The unstated hidden reason comments are disabled is because comment threads often dilute the message of the posting.
Threads can digress and accentuate negative, minor, or emotional/controversial distractions. The postings are a favor to the listing company. If that company prefers just "here's our message, contact us", with no risk of defocusing tangents, then that's the right favor for YC/PG to grant.
It may also be better to decide up front, "we'll give up the comments, good or bad", than to allow comments, and get a few that are so unproductively snarky they create a back-and-forth with critics, or that trigger premature speculation about things strategically left unsaid, or that prompt a later censorship decision (with its accompanying distracting gripetails).
Maybe ads are a type of content.
http://www.baekdal.com/publishing/why-traditional-ads-work-a...
The inevitable discussion about how we could just have an etiquette of not doing that, or a guideline for not doing that, or a new kind of flag button for that, or or or, will just serve to illustrate how much simpler a solution just not allowing comments on those posts is.
These posts aren't hurting you. My bet is most HN people actully think they're a benefit of HN --- early access to YC jobs! This is a fake controversy. Can we think about tolerating the YC job ads in terms of "least we can do for running HN"?
A posting anywhere with enough specific info to prompt applicant interest will prompt other observers to fill in the details. It's true for Google, Apple, Facebook etc. – so it will be true for self-identified YC startups who tout their impressive metrics as well.