BUT individuals have a constitution protection against compelled self-incrimination.
The San Bernadino killers are dead. So I'm not certain exactly how 4th and 5th Amendment rights would work. There's a good chance that the phone contains evidence that would incriminate others. But these others have almost certainly fled and/or destroyed evidence by now. So there isn't likely a imminent threat to be dealt with. So, what exactly is it that they need?