Apple has started enforcing a rule that says app developers have to offer the option of buying content within the apps for the iPad and iPhone, in addition to linking to a web store.

The immediate impact will be on Amazon's Kindle and Sony's Reader apps. Currently the Amazon app, for example, connects a user to the site when he or she wants to buy a book. Apple is asking that the sale be made in-app which means through the iTunes-based system Apple offers.

Apple rejected Sony's iPhone application because it allowed people to buy and read e-books through the Sony Reader Store, bypassing. Apple reportedly told Sony that all purchases had to go through Apple's App Store.

A Sony spokesperson said via email that the company is waiting for a solution that will allow users to buy Sony Reader Store content on their Apple mobile devices. The spokesperson said Sony is hoping to synchronize users' e-reader content between their Readers and iPhones as is possible on other mobile platforms. We opened a dialog with Apple to see if we can come up with an equitable resolution for both companies as well as our consumers but reached an impasse at this time. We are working on other solutions to bring the Reader experience to Apple mobile devices. Please stay tuned, the spokespersons said.

Apple did not respond to an inquiry for comment, but a spokeswoman told the blog All Things Digital that it has not changed its guidelines; it is merely enforcing existing ones.

The rule states apps utilizing a system other than the In App Purchase API to purchase content, functionality, or services will be rejected. Apple said to All Things Digital that it will allow Sony and others to sell content through their own stores, but will not unless they make that same purchase available at the app store.

Apple did not specify why these rules were in place.

Amazon did not respond to an inquiry for comment.

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