Robertson has value to OKC because they need someone to at least try to defend Durant, Harden, etc, and they don't really need offense from his spot. (they paid him before they got George, IIRC).
Smart's people can make the same argument regarding the Cs and Golden State. I'm guessing they are willing to settle for the $10m a year x three or four years, but the Cs would rather pay him the QO and avoid starting the tax clock this season. Assuming Smart's value this year and next is $10m a season, one year at the QO and three more at $11.2m still gets him to 4/$40, just with bruised feelings. There's always the danger of losing Smart next summer for no compensation, but they were willing to do that last summer with a relatively high draft pick, in KO.