You're assuming that storage is the main component of what Dropbox is, but that's not the case. The sync, bandwidth management, version control, and extensive integrations are what make Dropbox Dropbox.
Even if Amazon could reproduce all of the tech behind Dropbox, Amazon wouldn't gain much from competing with them. They'd lose Dropbox as a customer and be sending a signal that big users of their AWS platform are a potential target. They'd be competing against Google, Apple, and Microsoft in yet another market, and unlike those companies, they don't yet have a large consumer platform that integrated cloud storage is a strategic benefit to -- Kindles just aren't that large of a base.