You also get to take up to 65% of revenue, leaving authors with as little as 35%.
Yeah, it's a market / paradigm shift.
I know that I buy and read a lot more on Amazon/Kindle than I used to buy with paper books. Tended then to do a lot more visits to the library and used book shop vs. buying retail.
So less revenue per unit to the authors with more units sold vs more revenue per unit and fewer units sold. Not to mention making the market more accessible to nascent authors who might otherwise be unable to interest a publisher into doing a print & distribution run. Be interesting to see how the numbers truly worked out - though also kind of moot since if it wasn't Amazon it'd be some other entity.
Baen is an interesting example of a publisher doing a bit of their own thing in the ebook world -

Baen Books Science Fiction & Fantasy - Home Page
Baen Books. The best in Science Fiction and Fantasy publishing.