There's nothing special about the "Who's Hiring" threads. They're not even officially sanctioned. Somebody just posts a thread with that title at the beginning of each month, and since HN happens to be a good place to hire from, people post positions they have open. Any karma limit there would have to apply to all posts unless PG decided to make them an official thing.

The Jobs tab is something different — it's for job listings by YC companies, each listing gets its own post, and you can't comment on those at all.