I think this bug should be considered completely separately from how unwise it is to use a cloud service as the sole storage of important files. Regardless of the circumstances, losing user files against the wishes of the user is the absolute worst thing a cloud backup provider can do . Even for files that are deleted intentionally and unambiguously by the user, I'm astonished that Dropbox actually deletes the files…
On the other hand, keeping copies of the user's data after you say you've deleted it doesn't sound too ethical to me. I don't expect immediate purging, but I also don't expect potentially sensitive files to linger for months after they were supposed to be deleted.
I think a better system would be to warn about big deletion events (e.g. send an email) to allow the user to revert it within the 30-day period.