For those that don't understand the differences: I am unsure about the Mac, as I haven't used one in 2 years, but lockdown mode for iPhone requires a passcode to unlock. The constitution of the United States protects speech, and also guarantees the right to self incriminate. The two combined are why they can't legally demand a passcode. Biometrics are not a form of speech, and are not a form of self incrimination, so they are allowed.
EDIT: A small clarification: the government cannot compel you to say certain things, hence why freedom of speech is part of what I mentioned above. That is also why they aren't supposed to be coercing confessions (if they do and get caught, the whole trial gets thrown out).
