Except for 'reader apps' (those that sell digital content, basically) which Amazon is. Plus, Apple's rules are applied unevenly; Amazon is a giant WebView on Apple TV but it's disallowed for everybody else.
As far as I understand, what "reader apps" were allowed to do was to display content purchased elsewhere in the first place, which is orthogonal to being/not being allowed to link to external purchases, no?
These are the changes that Apple was forced to make, specifically referencing 3.1.3 (Other Purchase Methods) and 3.1.3(a) (“Reader” Apps):
> 3.1.3: The prohibition on encouraging users to use a purchasing method other than in-app purchase does not apply on the United States storefront.
> 3.1.3(a): The External Link Account entitlement is not required for apps on the United States storefront to include buttons, external links, or other calls to action.
The bit about the (formerly required in the US) entitlement is:
> Reader app developers may apply for the External Link Account Entitlement to provide an informational link in their app to a web site the developer owns or maintains responsibility for in order to create or manage an account.
They required you use a trackingless, generic URL that was unvarying per user, so you probably didn't run into it super often. Offhand, the Kobo app did use it.
