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iOS Apps with Dropbox Integration Running Afoul of Apple's App Store Rules
That functionality runs afoul of Apple's review guidelines, which do not allow signups for paid services other than those available through Apple's In App Purchase to be accessible directly from the app. The rule makes it more difficult for creators of subscription and other paid content to avoid Apple's 30% cut of In App Purchases.
As noted by MacStories, the Rule 11.13 Apple is citing in rejecting these application is not new, but developers running the Dropbox SDK version 1.2.1 which offered a way for users to get to paid signup pages were just recently beginning to experience issues with App Store review. As cited in one rejection notice sent to a developer:
11.13For its part, Dropbox has moved quickly to address the issue on an interim basis, rolling out a new SDK version 1.2.2 that eliminates access to both account signups and paid subscription packages directly from apps with Dropbox integration. In terms of a longer-term solution, Dropbox reported to The Verge that it is "working with Apple to come up with a solution that still provides an elegant user experience."
We found that your app provides access to external mechanisms for purchases or subscriptions to be used in the app, which is not in compliance with the App Store Review Guidelines.
Specifically, your app enables to user to create accounts with Dropbox and Google.
Based on comments from Dropbox representatives in the developer support thread, it appears the company would at a minimum like to have a way for new users to create free Dropbox accounts so that they can be used in association with the Dropbox-enabled apps using the SDK. Dropbox appears to be very willing to make sure users signing up through this mechanism are not provided with a means to sign up for additional paid storage capacity, thereby avoiding the primary Apple's primary objection to mechanism circumventing In App Purchasing.