Read the opinion, that was one of the argument used and it was overturned.
No, re-read the opinion; the court said he needs to ask for counsel and then abide by the “everything you say can and will be used against you in a court of law.” Ask for a lawyer and insist on your Miranda rights and everything will be fine. This guy never asked for counsel, he agreed to hearing his Miranda rights and then decided to talk/write anyway.
That is the right ruling. Otherwise, again the police read you your rights, then you can confess to everything on their books and well, they can’t use those confessions because you were told your rights. In other words, if you don’t apply/ask for your rights, then they don’t happen.