As already already been said there is no revenue sharing.

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.

AFAIK they do it without revenue sharing because it keeps customers using steam (running their client, seeing their ads, viewing their store, and being tied to the steam ecosystem). They don't need to turn a profit on the tiny, irrelevant volume of off-site sales.