So, if i"m understanding the OP correctly. What you are saying is , there are demo discs used to show off TVs, movies, etc, etc. Why are these not sold to the general public?
(And to be clear, you are not asking "why can't we make demo discs and sell them") right?
I think the reason they don't sell demo discs is because of legal issues... For example, let's say there's a demo containing an avatar 3D clip (which there is), and they decide to sell it to the general public. Now let's say that one of the actors on the movie complains that there is revenue being made off a product not discussed in the contract.. <-- not saying this is what will happen, but I'm sure there's a lot of legal issues like this.
Of course, there is the "fair use" law. But is it fair use if the item sells primarily because of a 2 minute demo? I'm not a lawyer so I don't know. i know that they'd rather either give them away with TVs, etc, or just tell the stores not to give them away... eventually they fall in the hands of employees, and other people.. but in the end, to sell them would probably require going through more red tape than it's worth......