"Why should I not be allowed to sell a tangible product that I've paid good money for?"
Because you haven't paid for the product. All you have paid for is a single licence to play the game on 1 machine. Just as if you work for a big company, you cannot go an buy a single copy of MW Office, then install it on 4000 machines. You need separate licences. (Flawed analogy I know, because with games you are no longer playing them, but it is about licences).
When you 'buy' a game or piece of software, you are not actually paying for anything more then a licence to use it. The fact that the publisher gives it to you on a CD/DVD is neither here nor there.
Regarding the subject in general: What if the new bestseller book comes out: Murder Mystery 5.
But they only sell one copy. To me. I bought it, read it, and leant it to someone else, who read it and leant it to someone else, etc etc etc. Are you guys SERIOUSLY saying that it doesn't hurt the industry?