Valve claims they don't sell games. They sell non-transferable licenses to use their games.
Say I bought a game, and enjoyed it, and one of my friends wanted to try it out. Can I loan them my copy, like with a book, or a movie, or a console? No. My copy of the game is irrevocably tied to my account. To do this, I have to give them my username and password and let them log in as me, in violation of Steam's EULA.
Say I bought a game, and have finished with it, and want to resell it on the secondary market. Can I list the game on eBay, and send it to the buyer? No. My copy of the game is irrevocably tied to my account. To do this, I have to try to sell my entire account, with everything on it, also in violation of Steam's EULA.
The doctrine of first sale ensures the right to resell a copy of a work. It mandates tolerance of the secondary market. That's why Valve claims that if you go to a store and buy a copy of the Orange Box, you're not actually buying any games, you're simply purchasing a license to use their software. You're purchasing access to Portal, not a copy of Portal. It's bullshit, and it's illegal, but short of a class-action it's never going to get before a court, and the Steam EULA even tries to get you to waive the ability to sue. Add to that the mandatory online activation of single-player offline games, so you have to hope Valve's servers decide to let you play the game you just bought? And that whole German Orange Box debacle? Fuck. That.
Essentially, this is Valve's business model:
http://ca.youtube.com/watch?v=31JNEVHZxO8