Kindle is the same as the rest. They all used to kick out to Safari. At first, this was because there was no in-app purchase API, so kicking out to the website

Kindle is the same as the rest. They all used to kick out to Safari. At first, this was because there was no in-app purchase API, so kicking out to the website was the only practical option. Then Apple added in-app purchase, but none of the ebook vendors wanted to use it (partly because of the 30% slice, but there were other practical issues as well).