>apparently so ill defined in law that 9 justices can't agree on it

That is not how the Supreme Court works. SCOTUS is a political body. Justices do one thing: cast votes. For any reason.

If they write an opinion it is merely their post hoc justification for their vote. Otherwise they do not have to explain anything. And when they do write an opinion it does not necessarily reflect the real reason for the way they voted.

Edit: Not sure why anyone is downvoting this comment. I was a trial attorney for 40+ years. If you believe what I posted is legally inaccurate, then provide a comment. But downvoting without explaining is ... just ... I don't know ... cowardly?