True, they could have kept it like this, but it would kill their profits. The tax brackets in the US are high, and Envato would have to decrease the commission rates to make a good profit. The way it is now, everyone can have roughly similar commission rates without Envato dying.
Although Envato will be paying tax on the authors fee, as well as the authors fee. I mean, I can’t confirm that, but if Envato are getting $5 in the bank from every $10 sale*, I don’t think it would wash with any tax authority if they were reporting that they only got $2. And whatever income tax an author may pay on that sale, it in no way reduces Envato’s responsibility when it comes to corporation tax. Well, in theory.
That’s the whole point of their shenanigans though. As the direct seller, it’s all the author earnings for which us authors are accountable for. What would be the point if indeed earning would be taxed twice? This makes no sense.