I'm sorry, but this reeks of complete ignorance. I don't think you're qualified to argue for SOPA.
the site is primarily designed or
operated for the purpose of, has only lim-
ited purpose or use other than, or is mar-
keted by its operator or another acting in
concert with that operator primarily for
use in, offering goods or services in viola-
tion of— [...some existing laws...]
I expect that at least one major copyright holder will use a similar system with SOPA, since there seems to be even less theoretical penalty for doing so than with the DMCA. So yes, I expect that the BBC might very well be blacklisted just because one used posts a link to infringing material. And then it will stay blacklisted until maybe the slow wheels of human review come to the consensus that it was indeed an error, and fix it.
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 :)