So, because Source has excellent physics, no other game can have excellent physics, because that would be stealing? Because Max Payne has bullet time, no other game should have bullet time, because that would just be stealin? Because Far Cry has a boat, no other game should have a boat, because that would just be stealing? Because... etc.
See how utterly retarded that mindset is? Valve took a really, really cool thing from another game, worked on it, and made it better. Isn't that sort of how the entire industry works?! You'll see that they probably put a lot more work into this, as having a player-made portal system is probably just a little bit harder to get working than a static system, so I think they've earned at least some respect. Anyway, are you saying that they shouldn't have done this, and we shouldn't have what looks to be an awesomely entertaining game?