It makes sense that Dropbox wants to avoid the popularity that would come from being an easy way to distribute unauthorized content.
However, takedown notices have been abused on YouTube to censor criticism, and dishonestly claim ownership. The problem could be even worse if what it covered wasn't limited to when links are posted publicly, but also covered shares between family members, colleagues, and friends.
While distributing backups from your DVD collection to a colleague may not make sense, and that isn't much better than posting public links, qwerta has a point when it comes to the potential for someone unethical to abuse the system.
I appreciate the challenges Dropbox faces with respect to copyright, and that it is easier to focus on making a viable business if a broad policy wastes less of their time on the issue. However, it's also a complicated when you've given a service read/write access to your hard drive—not necessarily because its reasonable to assume that Dropbox would take a legal risk like proactively deleting content on your computer, but because it's almost an unreasonable amount of trust you're granting them in the first place.
Anyway, maybe you meant, "Not all of your arguments apply to shared files."