I really like how Cryptomator works, and I think Dropbox could easily provide similar functionality, and perhaps in an even more user-friendly way, too, since they can just integrate it with their Dropbox app, rather than this functionality being in a whole separate application.
Dropbox is not an advertising company like Google. They still have some of the highest fees for cloud storage around. So why do they care about seeing what's in people's files? Why not allow people to encrypt the files locally before uploading them?
And lest we forget, in the PRISM slides, Dropbox was mentioned as "coming soon". So unless they want to admit they are already part of the PRISM program, then what better way to dispel those rumors (not made any better by getting Condoleezza Rice on their board) that they are cooperating with the NSA.
http://www.zdnet.com/article/fbi-nsa-said-to-be-secretly-min...