All AH transactions they require additional posting should be done so through PM or some other form of communication.
This is to prevent from the replies that many ppl get in their WTS/WTB thread: "OMG....your selling that for way too much. Thats ridiculous." or "You seriously think someone is gonna sell you that for that much?".
Those kinds of replies are simply PC+1's that serve no purpose what so ever to the original intent of the thread. So, if nobody can reply then that takes that out of the equation.
It isnt that difficult to get ahold of the person hosting the thread through a different means of communication other than posting a reply on the thread. Yes, true, it is not as 'convieniant', but convieniance has been sacrificed in order to allow a clean, garbage-post free thread to exist without the constant management of a mod.
In short, it helps more than it hurts. And we all know that sometimes you have to give a little in order to get a little.