And you think proof reading is the only difference between a amazon digital book and a traditionally published book?
Don't put words in my mouth. I handled the cover design for my books, though the publisher did offer to do it and did convert it into a format that was better suited for a book cover. I wrote technical books, so the publisher also paid for technical assessments to make sure I wasn't wrong along with the editing. Those are very valuable services.
Simply hosting a few MB file (the book) and processing a credit card payment is not enough to justify charging 30%.
But hosting a few dead trees in a warehouse is worth 50% of a book? Amazon is the 300lb gorilla in book sales, so they'll charge what they think they can get away with. If another vendor comes along and gets the name recognition that Amazon has and only takes 20%, then Amazon may change their ways.
Don't get me wrong, I think it's great that authors have lots of options to get their work out. But the choice is they either get about 5% and have a huge infrastructure behind them to help get their work out, promoted, and looking nice, or doing most of it themselves and getting 70% and hope they have an underground hit that lots of people buy.