Why not B?

You are on the right track! “For its own sake” implies that they advocated for freedom of speech for no reason beyond it being basic human right. The first sentence states that, “freedom of speech is not only a basic human right”, and the passage goes on to give us other reasons to support freedom of speech, such as the public dissemination of good ideas. Thus, the “for its own sake” part of the answer choice makes it incorrect.