Most likely the average HN reader is well aware that dropbox must have access to read your files. But possibly an average joe who doesn't read the TOS and who doesn't think everything through would assume that private storage is private to joe only and that the advertised encryption extends beyond transport to storage.

Because you can use Dropbox as a raw storage system by encrypting stuff on the client ?

I avoided Dropbox. The news about FireFly just remembered me why I did it.