Have to say, yes, it is a threat. However, we do have precedents of similar situations. Facebook and Youtube actualy host content as content. DNS servers and internet provider can distribute illegal material in so far as that they allow or help you to access it. The material exists regardless elsewhere.
Now, what do Witnesses do? If i halfway understand it, they document the blockchain. A blockchain that exists regardless of what they do, just like a server with CP exists with or without your internet provider. I would say that should legaly fall in a similar category as internet provider.
Of course, the difference is that in the case of files on the internet, they can demand that server to take it down, while the blockchain isnt an entity you could prosecute. The witnesses are and id wager that at least some politicians would see it as a good idea to go after them at least for a while.