Sure, would be nice to have statement from the devs about this kind of issue. I don't see this as a huge issue personally, but I can see where you're coming from.

Webhosts, for example, are not responsible for the content that their users put on there. Internet forums like reddit, etc. neither. They should do their due diligence in supporting prosecution (logging IPs, etc.), take down offending content when it's discovered, but that's where it ends. These rules can be applied to ISPs, image hosts, and so on. Dealing with 3rd party content is nothing new.

Sia is the same thing, it's just a medium, a protocol, nothing more. Not to mention that there is no one entity that you could "sue", you would have to sue the entire userbase, or the developers.