There really is no difference between syncing and backup. Think of the backup/restore cycle as a sync and it will be clear. Backup is one way sync, restore completes the bidirectionality of sync.

Sync doesn't necessarily have the capability to destroy files, rsync has a switch to delete files that are locally deleted. However Dropbox is supposed to be rsync + rcs so this kind of problem is supposedly easy to fix by simply reverting to a good previous version.