Updated: Apple Forces Kindle, Nook, Kobo To Yank In-App Bookstores

See ya, Kindle app store: *Apple* has begun enforcing in-app purchasing restrictions for e-reading apps, with the Kindle, Kobo, and Nook and Nook Kids apps falling into line by removing bookstores from their iOS apps.

Apple (NSDQ: AAPL) announced in February that any content available through apps must also be sold via the iTunes store–where it would be subject to Apple’s 30/70 revenue split–and that external purchase links must be removed. It revised those rules slightly in early June to say that app users could still read content purchased elsewhere from within their apps, as long as external buy links and buttons were removed. Publishers were given until June 30, 2011 to comply, and now Apple has begun enforcing its new guidelines.

Here are the major e-reading apps affected so far:

Kindle: The Kindle store was removed from the app today.

Kobo: The Kobo store was removed from the app on Saturday. Kobo outlines the changes on its blog: “With this change, iOS users wishing to access their Kobo account, browse the Kobo Store, and purchase books will now need to go to Kobo.com. You can still browse Kobo’s selection of 2.4 million e-books, shop and access your account! Go to Kobo.com by directly opening and using their Safari browser or using your favorite web browser on your phone. Be sure to bookmark the store for future visits!” The post also notes that Kobo’s apps for other platforms–Blackberry, WebOS and PC–retain their in-app stores.

Kobo CEO Michael Serbinis told the WSJ that “half the company’s iPad and iPhone customers already buy directly from the company on the Web because it’s more convenient. ‘But this will inconvenience those customers accustomed to buying their books directly from our apps on Apple devices,’ he said.”

Updated: Barnes & Noble (NYSE: BKS) Nook and Nook Kids apps: The bookstores have been removed from the Nook iOS app and the Nook Kids iPad app. The regular Nook app has not yet been updated.

Google (NSDQ: GOOG) Books: A Google spokesperson e-mailed me to say that he can’t comment on whether the Google eBooks app’s temporary removal from the iTunes store was related to Apple’s new in-app purchasing restrictions, but “we can confirm that it’s available once again.” The Google eBooks app has been removed from the iTunes store completely. Click on the “Download” link here and you’ll get the message: “The item you’ve requested is not currently available in the US store.” I’ve reached out to Google to ask if the app will be back soon sans bookstore and will update this post if I hear back.