I agree with Rob, it is to do with the fact that these things are designed to meet ADRs, and the cops know it, rather than your modded import, which MAY WELL meet the regulations, but they assume they don't until you can prove otherwise.
It's like those CAPA blowers and the like - massive performance increases, but the kits are designed to meet Australian emission specs, so the cops don't tend to look at them too hard.
CSV may have been a better example than HSV for the other guys too, cos they are both a car manufacturer and a "tuning shop". They will build you a car, or do yours, but they have to ensure everything meets those lovely design specs again.
I'm not sure to what lengths you can go to getting your modified import certified as meeting all specs, and it would probably be costly, but it could be worth a look, then at least you would know what you were driving was legal, and could have some evidence to back it up - knowing that if they were a prick and defected you anyway, that you could simply get it easily removed with a trip to the RTA.