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The Next Web reports that Apple has rejected a number of apps with Dropbox integration.
Apps with Dropbox integration can either authenticate through the installed Dropbox app or opens a webpage to let users log in to their Dropbox account.
As seen in the screenshot below, the problem with the web-based login is that it includes a link that allows users to signup for the service and purchase storage, thus bypassing Apple's App Store, which violates App Store review guidelines.