Good job infinite loop, you thought it through and came up with the only really good question. They're obviously going to sell it through steam, that's been the whole plan and in my opinion a lot of the hold up, so why can't they sell it on August 4th if that's the day it is shipped for Gold? Answer, there's really no reason at all why they couldn't...
That question has a very simple answer to it and yes there IS a reason why they couldn't just ship it on Steam as soon as the game is gold. The answer is in their contract with the producer, VU games.
Do you honestly think ANYONE would agree to package and ship a game like Half-life 2 if it were released to the public a month before the boxes would hit shelves? Of course not. Half their customers would buy it online and VU would lose out on a ton of potential profit. Valve stated in interviews that their agreement with VU games is that they would release it on Steam the same day it was released in stores.