The constitution of China guarantees freedom of speech, who gives them the right to violates human rights?

well, besides the problem of law enforcement in china - i think the trick is that somewhere in the constitution its also said that every human right, actually every citizen's right at all, can be overruled when "the safety of the state is threatened".
and guess now who's to decide when thats the case... ;-)

As does the United States

No person or entity gives any government the right to do anything. Governments moree or less reflect the will of the people and the people more or less get the government they want.