The British version assumes that any reasonable and innocent person will willingly co-operate with the police, and that the guilty person will also do so in order to mitigate any eventual sentence. Just as the courts are supposedly more disposed to deal leniently with an accused who pleads guilty.

The American system assumes that the police are part of the (du-du-DAAAA!) Government, and as such determined to take away your liberties (and sacred right to kill someone in cold blood with your shiny new Desert Eagle).