Factorio: Space Age continues the player's journey after launching rockets into space. Discover new worlds with unique challenges, exploit their novel resources for advanced technological gains, and manage your fleet of interplanetary space platforms. For more information about what comes with the expansion, check out our Space Age content page.
They have no control over it. Steam uses regional pricing, but Humble uses straight currency conversion.
Feel free to buy it from Steam. You get both versions (Steam and stand alone) either way and both options equally benefit the devs. Steam might even be prefered, Kovarex mentioned they benefit greatly from a big hype on Steam in the first days.