Given the nature of ads on HN, not offering a comment section for folks to ask for clarification in promoted posts devalues the ad itself. If a YC2019 company is hiring, and their job page is lacking, the comment section can help get it improved. If someone makes a tech announcement as an ad, then all we've done is sold a spot on the front page without the freedom of discourse.
Whether it's paid placement or user-submitted, the community should have a voice (and if that voice is one of dissent and disapproval, that's an important signal that is currently unavailable to those deciding whether to sell that front page spot to X or whether to let Y have it instead)
