My opinion: They don't want ownership of content. They would become responsible for it. They don't want that, because not even they have real control over the data. So you have to confirm in the ToS, that you remain responsible for content you posted, but the system gives you even less control over "your" content than witnesses have. So you are f'ed from both sides. Maximal responsibility, minimal rights. In theory, one single post that somehow contains top secret materials could outlaw the whole blockchain. If you think, well, our witnesses are multinational, just wait until there are some globally enforced laws.