Apple is never consistent about rejecting apps. The reason is what matters here, not the number rejected (as long as it wasn't a fluke, and since it was multiple devs I don't think it was).

They very much are being rejected for using dropbox. You can be more specific and say it's for letting the user log in to dropbox but that's it. Apple is blocking it because if you spend long enough wandering around the site from the login point you might find a place where you might be able to purchase something.

Logging in to dropbox is a core part of using dropbox and to suggest otherwise to call the title into question is very strange.