Only if you charge the same as Steam, i.e. you can't pass the 27% (still pay payment processing) savings on to customers, unless it is a limited time sale.
Steam only allows it because they want to be one centralized hub with more and more people locked in through social features, existing libraries, recommendation traffic, etc., but it is only allowed if it isn't price competition so that consumers can't feel the weight of the 30% directly.
No business plan makes sense if they give away their product for free without taking any form of revenue. Casting that as evil is odd and misleading. That they allow devs to generate codes that give steam no revenue at all is pretty incredible.
Well yeah... whether you sell your game on Steam or via Steam codes on your website, the soft-, hardware and business infrastructure in the background handling the sale and distribution of your game is the same. So why would they let you use their infrastructure without letting them have their cut? It wouldn't make any sense.