Oh, but they broke the ToS, that means they're not following what the web site's purpose is? Well, now maligned websites just need to have a ToS that says "don't do bad stuff", and they're fine!
See where I'm going with this?
For one thing, those files aren't publicly available by default. If they are publicly available, then the Dropbox team is probably already monitoring those files. If they are not monitoring those files then (this part is controversial...) maybe they should be investigated for copyright infringement, as long as they maintain due process.
I doubt that they are. Copyright is a matter of permission. How would any third party know who has permission to do what?
Due process is good :)