Well you could argue that steam just wants to make sure you don't exploit it as a pure marketing site, which is understandable imo.
Viewing post in Update News and future pricing. comments
I mean offering different versions of the same product is a mainstay of the gaming industry, the amount of console-exclusive or store-exclusive DLCs is wiiiide, so for Steam to go "you can't offer an inferior version of your product for free" is purely done to try and maintain their market share over actual philosophies or ideals.