Imagine going to Walmart and telling them what you want them to give you all the revenue. Walmart will tell you exactly what they want to pay you, or you can leave.
Maybe 30% is too much, but 0% is a terrible idea.
In the old days before the web I had to pay up to 70% for distribution for my apps. If I was lucky I might get my cut 3-6 months later, plus I had to pay for the packaging and shipping to the warehouse and take it back if they didn't sell enough. 30% seems like a bargain.