Once the police read you your Miranda warnings, you can remain silent and it can't be used against you. The issue in this case was the defendant's silence
before the police read him the Miranda warnings. (The prosecutor argued to the jury that the defendant was guilty of manslaughter, not just reckless driving, because he smashed into another car and never asked if the passengers in it were OK-- thus, according to the prosecutor, demonstrating his "indifference to human life".)
This is a very unsettled area of the law; the California Supreme Court split 4-3 on the issue, and it might well wind up at SCOTUS.