Freedom of speech does not mean you are free to say anything to anyone and never face consequences for your choices. The concept of freedom is to be free from the government officially and legally restricting your right to publicly address your grievances.
China's citizens do not have this freedom. Just because taxi drivers are not afraid to express themselves in the privacy of their cab to other citizens does not mean they wouldn't fear for their livelihood to say the same thing on a stage where it would actually make a difference.
True freedom of speech is this:
In America, anyone can stand in the middle of a public square and shout whatever they want about the government and the police will do nothing. Other people are then allowed to also shout and demonstrate that that person is an idiot, or maybe right, but the government never gets involved.
In China, any demonstration like this is swiftly punished by authorities. On an exchange trip I went on about ~10 years ago an American college student decided it was good idea to shout "Free Tibet!" over and over in the middle of Tiananmen square. An unmarked white van appeared in < 5 minutes and he was gone. He was unharmed but that evening he was on the first flight back to America.
People may hate you and retaliate against you in America for what you say, but they cannot and will never arrest you.
That is freedom of speech.