[anotherman554]

"If you tell a police officer that you did a crime, then your words are admissible as evidence against you."

This is because the people who wrote the evidence rules believe nobody would admit to a crime unless they are guilty. So it's a hearsay exception.

The exception isn't meant to be a sinister trick to treat you unfairly, it's meant to lead to the right people going to jail and the right people not going to jail.