One of the things to remember whenever you buy direct from the publisher (be it online, retail or mail order), is that they will usually sell new releases at RRP.
The reasoning is very very simple, it upsets the retailers who will shift most of the boxed copies, if the publisher undercuts them (meaning they don't even get a chance to make a profit and get left with loads of stock).
Which in turn upsets the distributor, which leads to the publishers titles simply not making it into stores (or only in small quantities).
However, some publishers will offer exclusives if you buy from them (a few book publishers will do signed copies, some DVD distributors/publishers who deal direct with the public will put in a LE item or something).
So if the US price for a game is $50 boxed, then Steam probably can't sell it for less than that for a certain time (even on their own titles), without breaking the contract they've signed with the company doing physical distribution.
It used to be the case that you generally didn't get much (any) discount on the RRP when buying new games, however now with online third party retailers (some of whom are huge) able to cut most of the retail overheads out of the equation for physical items, we've got used to having an instant 25-50% off new releases.
I guess what i'm saying is that Steam probably can't lower the price for their online delivery version of the game, without losing their ability to get stuff into the shops (where a lot of people still buy their games, even without discounts).