Is it unreasonable for my account to have been hacked and used to hold illegal material?
There are accounts of mine that I don't even remember existing.
Multiple use multiple devices of mine from time to time.
Should my dad be hauled to prison when I use his computer, and I catch a virus that stores child porn in his dropbox account from 5 years ago?
I wouldn't necessarily say unreasonable. Incredibly unlikely, though.
The question is whether there could be reasonable doubt that, given there is CP on an account, it was put there by the account owner.
What we don't know is what proportion of Dropbox accounts that contain CP had that CP planted by a hacker. If a significant enough proportion of CP on Dropbox was not put there by the account owner, it's reasonable to at least consider the possibility.
It doesn't have to be an attempt to frame the account owner: somebody who wants to exchange CP with other people might well use a hacked Dropbox account to do that, in an attempt to cloak their identities.
If it's the case that someone illegally accessed your account and stored child porn on it, there would presumably be evidence to show that illegal access, such as IP addresses that you don't use that were used to upload that porn.
What if it was (somehow) accidentally uploaded byy little brother who torrents?
Technical evidence, is difficult to associate with meatspace users.
I'm not trying to give criminals potential defenses---
I am trying to prevent people from going to jail for malleable evidence.
There's still the possibility that some malware on the user's laptop uploaded the files and then deleted itself, but if that were a reasonable excuse, you could use that to get out of basically any computer-related crime.