Keep in mind that the requirement for in-app / in-store purchases currently
only applies to subscriptions. The
official announcement strictly discusses subscriptions.
Purchasing content on a non-subscription basis is not affected.
This is the same as how Amazon treat magazines -- they take a high percent of the cut; they refuse to share customer data with the magazine publisher; delivery is locked down.
So, the Kindle app won't change, unless they added their magazine offerings to iOS apps without my noticing.