Sans the "Shop in Kindle Store" link, users can still purchase books outside of the App Store -- on Amazon's Web site, for instance, or through a non-Apple Kindle app -- then have them loaded to an iPhone or iPad.
Amazon must also implement Apple's single-click in-app purchasing of content; Apple would skim 30% off the top of all such purchases, however.
Apple did not specify a deadline that developers must meet to modify their existing apps, and Muller did not reply to a follow-up question asking for that date.
The latest news adds to the controversy sparked earlier this month when Sony announced that Apple had rejected its e-reader app. At the time, Sony said it had "reached an impasse" with Apple over its in-app purchasing rules.
The first publication to announce that it would offer in-app subscriptions was News Corp., which launched its all-digital The Daily newspaper on the iPad two weeks ago.
After The Daily's launch, several analysts and pundits predicted that Apple would debut in-app subscriptions today because the newspaper's two-week free trial -- sponsored by Verizon -- was to expire on Feb. 15.
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