It doesn’t really matter what the contents of the file are if they are extremely hard to trace and also unrecognisable from the point of view anyone but the owner. The point is that you get paid.
There’s no such thing as a 100% guarantee, but it’d be extremely costly to do so, since the network is in a constant state of flux and has multiple layers of encryption. In addition, it’s also rather useless, the moment your system goes offline the network makes another copy and stores it randomly on another system, so it’s impossible to remove data from the network that way.
Also, what would they prosecute you for, even in the case of child porn? That there is a random shard of encrypted, unrecognisable data that was originally a set of random blob of pixels (perhaps even merely the ceiling of the room) of a picture that contains child porn that was uploaded by some random dude from the other side of the world? That won’t hold, not even in a US court. It would be impossible for companies like Google, Apple, Amazon, Microsoft, etc, to run cloud services then, since anyone can upload the most disgusting and illegal content in encrypted form to it without their knowledge.
This over-focus on child porn is extremely unreasonable. Holders of child porn can already use encryption and store their shit anywhere they want, either locally or in the traditional cloud, and nothing can stop them from doing that. You can’t ban mathematical equations.
At the same time, practically the entire internet is vulnerable in terms of security on almost every level, it is a huge, world-wide problem. Just look at any of the studies warning that vital national infrastructure is extremely vulnerable, or what happened to Sony, or the extent of secret service spying, not only for security reasons, but also business espionage.
The abuse that can be solved by a safer internet platform like MaidSafe completely dwarfs the harm that some nefarious people can hide behind secure data storage and secure internet communication.