Yes, I think it is unfair to pull out after accepting an offer because there is a lot that goes on in the background for the buyer in preparation of the purchase and although it might seem like they have not lost money or anything and you are usually advised not to make any commitment until contracts have been exchange, it is not that straightforward. Up until offers has been accepted, it should be fair play to allow for bidding and it is the sellers choice to accept the highest bidder or whoever they prefer to sell to. However, once the offer has been accepted, I believe it is only fair to honour the agreement.
This also should go both ways. I think the buyers also need to be made to put some form of deposit so that if for whatever reason, they cant make the mortgage, then that fund goes to the seller. It is unfair for a seller to wait on a buyer only to be told after many months that the deal has fallen through.
Some form of reform is needed to protect both buyers and sellers.