They probably pass because you just pay off the copper. Heard it a few times - 80% of the Newcastle cops are corrupt. I would be very, very surprised to find that figure vastly different anywhere else in the country (except for a one-horse town, and then I'd expect it to be 100%). Fact of life - they're in a position of power, and power corrupts.
My Wing has factory exhaust. No extra noise - and since I hardly rev the engine at all, they'd be hard pressed to notice me by the noise alone.