The issue is the way Dropbox’s new SDK handles the authorization of third-party apps: it sends users to a page in Safari where they can grant access. However, that same page also allows new users to create accounts, after which they could drill down through Dropbox’s site and upgrade to a paid account.
…and thus, Apple is rejecting them because you’re not allowed to have a link in an iOS app that allows for external purchases to be made unless it’s also available as an in-app purchase.
Federico Viticci writes, “Before we dabble in speculation about Apple wanting to ‘kill Dropbox,’ I suggest we wait.” True enough, and to a degree I understand Apple’s general position here—it’s kind of like letting companies put up ads on WalMart shelves for the sales they’re running at Target. But the rejection of Dropbox-enabled apps shows the flip side of this: prohibiting this kind of linking makes the user experience of certain iOS apps objectively worse. Apple is effectively saying they consider this an acceptable tradeoff.