While you can't argue with Valve's retail policy, region locking CD keys is kinda bullshit. PC games are supposed to be able to work in any region no matter where you buy them. Region locking is a form of DRM that should be left the assholes that run the movie companies. They need to keep that shit out of PC games.
I want to agree with you, but not all countries are equal. In order to sell games and combat piracy in other countries they have to charge a lot less. Some people just flat can't afford their currency's equivalent of 50 USD. And it really isn't fair for me to go their and buy, exploiting the system. We hate it it when people come to the U.S. and exploit crap here.
The only thing holding the $15 deal up in my mind is that it said "region free." Should be totally legit if that's true. And if not, you would've gotten Steam's standard region warning.