Brain surgeons makes mistakes, people die. Pilots makes mistakes, people die. _Everyone_ makes mistakes. There is not one single person on this planet who is perfect, human error, this includes the employees of rsync.net, or any other company for that matter.
Today it happened to dropbox, tomorrow it happends to Visa, Bank of america, Amazon, rsync.net, <insert X company here>
Its just life, learn to deal with it. If you seriously have seriously confidential stuff, you're probably intelligent enough not to "upload" it anywhere, much less some file service with millions of users.
The more users the more exposed it is, human error. No encryption or system will ever protect against it, at least until we have true AI, and yes you also make mistakes, now matter how stupidly simple or complicated they are, nevertheless you do.
And dropbox, don't get a sad face because of this, just look at Sony or whatever, then smile.