Re: I don't get it...
It is a principle of both UK and US law that no-one can be compelled to give evidence against themselves and it always has been.
The idea is that the prosecuting authorities should be able to make a ca,se that stands up on its own and so does not have to rely on a confession, which can be extorted or made up. It may seem unlikely that someone would confess to a crime they didn't commit except under duress, but there are plenty of examples of that happening. There are far more examples of false confessions being forced out of suspects.
This is a difficult area and I have to say that, on balance, I don't agree with the Supreme Court. The police aren't asking the suspect to incriminate himself but to facilitate the analysis of evidence. I don't think that is the same thing. It doesn't take a genius to work out why he refuses to divulge his passwords/encryption keys, but until he does, no-one will know for sure.