Dude, of course there is a difference. If you sell a physical good you do not have that physical good anymore. If you sell a digital copy you can keep selling that digital copy because you do not necessarily give it away or delete it.
Saying that there is no difference at all between the two is silly.
To be clear, I am not saying this justifies anything regarding copyright, but it is a difference if you can sell something over and over again or just once.