If points themselves were a product, then maybe. But points are a promotional tool. The moment you make points a salable product you make them a surrogate currency. That's the thing about points. They don't go off and they don't physically exist, but they are there to trade for goods or services. so in essence they are a trade mechanism. If you make it exchangeable for dollars in an open market, - as you do when you allow them to be openly sold, it becomes a surrogate currency. This is everything the airlines don't want and would be a major regulatory problem for them.
I don't get this whole... monopoly thing. I know this sounds repetitive, but it is a promotional tool for the airline. Of course they are going to have absolutely control on their own promotional tool.