Valve games currently require Steam (on PC) but can be bought anywhere. Making HL3 a Steam Box exclusive would alienate the vast majority of their fan base and kill all the good will they've built over the years.
The third and fourth comment are directed at the second comment, but are displayed in reply to the first comment since there's only one level of replies. Confusing.
Also, instead of linking to the comment to show this, I had to take a screenshot.
