Earlier quoted context omitted.
Steam lets developers sell keys without the tax using their own systems. In addition to this you are allowed to list your games on other storefronts for PC.
Do you know if there is any revenue sharing happening in the background in that model?
Though there is fair-use policy that Steam users shouldn't be given worse deal. E.g you're not allowed to only run 30%-off discount exclusively on your store without offering same discount for Steam store users sometime later.