The SF type of free speech is just of a very different kind. In China the wrong political ideas can cause you trouble with the government and potentially in jail. In SF the wrong ideas or words can lead to rejection from your social circle and loss of your career. To a lot of people the latter is just as scary, if not scarier than facing a hostile government. A year in jail doesn't sound as bad to some as becoming a social pariah.

And the government is at least sort of predictable, and is unlikely to make you disappear without any warnings. Social circles and careers can flip at a moments notice based on criteria no one is exactly sure of. A less than favorable opinion on any racial or sexual minority group or gender? A too favorable opinion on something sexual? Some comment regarding a woman's appearance? Support for a certain political party? Not thinking some individual woman/minority is amazing? Desiring the (il)legality of guns? Desiring the (il)legality of abortion? Other opinions on healthcare? Expressing a preference to (fe)males for some occupation? Acting on the belief it is okay to flirt in a workplace environment? Believing 'political correctness' is bad? Thinking human engineering is a good thing? Being for/opposed to the trans movement? Having some religion? Not having a religion?

You never know which one of these is capable of killing off your career/social group. In your average environment today, you really just can't tell which of them will set off the bomb. So you're silent on all.