Tea cup Chihuahuas are legal as pets despite being often smaller that a rat, so it isn't size that's the issue.

Cats and dogs are legal pets despite the feral populations and the damage they do to the environment.
So how can a tiny animal confined by a cage majority or under human 'control' of the time be illegal?

A rat can be trained better than a cat, fish and bird so that can't be it.

Rats are often cleaner than most pets of all sizes so that can't be it.

There are more dog attacks every year than rat attacks.
The reasons given to not have rats as pets, all apply to most if not all other Domesticated animals.
These types of laws are unfair and unfounded. Make rats legal every where allow a national Breeding society like every other animal.
Make your shires money of the registration of rats, like dogs and cats, not the destruction of these cute intelligent creatures.
I can promise you with the number of rat lovers in WA, shire pockets will benefit greatly, and so will the rats with legal regulations of their care in place.