The thing is that if the user doesn't have the Dropbox app installed, then the authorization for purchase of additional storage goes through Safari, which might eventually allow them to do it directly on Dropbox's website, if he or she chooses the desktop version option.
This, according to Apple, would constitute "sending user to an additional purchase", a punishable offense. A developer received letter of rejection from Apple for this exact same reason, stating:
Reports are that Dropbox is working actively with Apple to remedy this issue. The Financial Times recently reported a big success of its HTML5 app that circumvents Apple's 30% subscription fee cut, so we might see a lot of publishers choose that route in the future.