Are you really saying "If someone has done nothing wrong, why shouldn't they co-operate"?
I don't like Mr. Parry's column, but he is basically saying what every other defense attorney would say, "Do NOT volunteer information or answer questions without an attorney present." There is no obligation to answer questions from the police or prosecutors. Further, if you do so, "anything you say may be used against you".
No one is obligated to prove their innocence to police or prosecutors. It is up to police or prosecutors to show there is enough evidence to prosecute someone for a crime.
If someone wants to co-operate with the police or prosecutors, they should do so with their eyes wide open.