MS has probably a custom USB ID string embedded into its own wireless USB adaptor, and the console's firmware will likely only accept that custom ID and not any other wireless USB plugs... This is probably as much a programming issue as it is an economic one. It's unlikely that all wireless USB chipsets are register-compatible, and also, by locking out all competition MS can maintain its himalayan-high profit margins.
Of course, anything that attaches through the ethernet socket would work as long as it follows specs, since a wifi adapter is invisible to either device on the ends of the ethernet cables...