I really don't see why not, either way you have to use steam to play (probably put in cd key), so why can't we pre-load the game aswell? It's not like we'd be playing it for free.
Because the non-steam retailer you bought the game from doesn't know the CD key inside of the box, which is the only way they could let you pre-load it through Steam.
Your comments are somewhat misleading regarding Brinks means of distribution. Brink, no matter where you buy it (online or brick/mortar), uses Steam. If you buy a hard-copy you will likely be able to sign onto Steam, and use whatever code has been provided, to install Brink via Steam if you wish and to not use the provided DVD at all. With this in mind, it would have been nice to allow anyone to pre-load and then use there activation code on Tuesday when they get their hard-copy.
It may have been nice but it would have also used Valve's bandwidth unnecessarily if people downloaded it and then bought it. One of the reasons to use Steam is being able to pre-load games, it wouldn't make sense if they negated that advantage by allowing everyone to use it instead of people who bought the game through Steam.