I do not recall ever having seen a thead discussing the fact that FlyerTalkers are restricted from offering airline/hotel specific coupons on the forum to which they relate.

Do any such threads exist?

Quite to the contrary, as has been pointed out, it is quite common for FlyerTalks to offer program specific coupons on the appropriate program forum. Why all of a sudden is this a problem?

Again, specifically how is FlyerTalk enhanced by this restriction? Posting coupons on specific forums in the past certainly hasn't "cluttered" them. What other objective could be achieved by this practice?