If you think 70% commission is high, you should look into the deals authors get from publishers. Typically author gets low single digits, the shop gets around 50% and the publisher gets the rest. Sometimes the author gets only a fee, then zero commission.

30% is the norm from Amazon anyway, though I agree the terms are onerous and a world in which Amazon (or any other large corp.) dominates distribution would not be a pleasant one for authors. The nice thing above the that though is that the Internet means that monopoly on digital distribution can be broken relatively quickly by a superior solution.