I haven't read the case that the EFF is citing, but here's Wikipedia's summary of the legal reasoning, which is what precedent consists of:
The U.S. Supreme Court in its review asked a more basic question: With the loyalty oath has California chosen a fair method to determine whether a tax exemption claimant is in fact someone to whom the criminal acts specified applies. In other words, though it is reasonable to deny a claimant a tax exemption if the claimant is involved in a criminal behavior, has the state arrived at a mechanism which demonstrates the criminal behavior?
The court ruled that because the state requires the claimant to show they are not advocating state overthrow and hence are not criminals within the applicable laws, the loyalty oath requirement to obtain the tax exemption is unconstitutional. The burden of proof for a criminal action rests on the state and not on the individual private citizen. In other cases, the Supreme Court has upheld the constitutionality of loyalty oaths requirements but those involved public officials and not private citizens.
If Wikipedia's summary is accurate, this case is not very convincing as precedent for the current Apple cases.
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HackerNews is rate limiting me, so here's my reply to the below:
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My point is that when they sign and publicly release updates, they are in fact engaging in speech (protected speech, I would emphatically argue). They are telling all of their users that this update is trustworthy and authentic.
When they comply with the current court orders, there is no such communication to a human being. Apple is not communicating anything to anyone. The government of course knows that the update isn't authentic, they're the ones compelling them to produce the update. They are hacking a machine, not engaging in speech.