I doubt they're actually paid, but it's wise to remember that Hacker News is intimately connected with Y Combinator. It's partly a vehicle for the companies to get feedback/criticism, and thus to some extent, promote their products. YC companies are allowed to post job listings which aren't able to be flagged or commented on. It's just a part of the culture here. Whether it adds value is a personal matter. For me, it generally doesn't, but the overall discussion is of high enough quality that the small amount of noise those generate is worth it.