I think a lot of forums do it out of respect for other people listing ads. Consider that every time someone makes a comment on a thread in the classifieds section, it goes to the top.
Since you're familiar with archerytalk, you'll note that a new thread is on page three within 30 minutes. I actually prefer not being able to comment on classified threads for this reason. When I was looking to sell my bow, I gave up after the first few "ttt" posts in the first few hours just to keep my ad somewhat current.