You are indeed correct that most Torrent Sites are mainly piracy distribution hubs. However, torrent sites are not indicative of torrent traffic by _Clients_. *
From what I understand, Blizzard uses torrents to spread patches for World of Warcraft. It is also used, I believe, in Steam as well.
* I would also argue that, although copyright violations, transfers of TV shows are already done via the main distributors' websites. Other than where the source is from, I do not see much a difference.
I would also argue that piracy itself is a response to market failure. When it's easier to get working media (notably cracked programs and music/movies/shows) via a 3rd party distribution than from the source, there is _something_ wrong. Many times, it is because of "We wont sell to that country until $later", or "Our antipiracy software wont run on your computer", or it just is infeasible to find it. But essays have been done on this topic alone.
My guess is that Dropbox cares about its image and wants everything piracy related to be kept at arms length. It’s not their business model. They want to be a trustworthy service everyone – from nerds to their moms and dads – wants to use.
I would argue that the prevalence of piracy harms, for example, Rapidshare’s image as a serious filesharing service. That’s not an issue for Rapidshare because piracy is their business, but it isn’t the business of Dropbox.
This is consequently not so much about the nature of piracy and much more about the image of Dropbox.
Whether or not Dropship would actually be a good tool for piracy is very much an open question (and one you can certainly argue about), Dropbox seems to think it is.
(I also want to note that even if only one percent of all torrent traffic is piracy related, it’s still wrong to compare it to knives. There are so many knives in the world, the fraction of knives which are used to harm people must be infinitesimal. And, to clarify something else: I would be vigorously against any legislative attempts at banning torrents. Legislatively, that’s just not the right way to go. But that’s the law and Dropbox is no government.)
Most of the time, a knife is used for cutting food and it can be used for murder.
Most of the time, torrents are used for piracy and they can be used for legitimate and legal files.
Search for "torrent" on Google. The majority of the results are search engines for pirated material (and in many cases, direct links to the torrents).
See the difference?