As we all know, you need a blue slip when bringing a car interstate into NSW. This is to make sure the car adheres to the safety guidelines in this state for a safe on road car.

My question is, if the Magna TH Executive is only supposed to have 15 inch rims as a standard, then why is it that we can purchase these rims for our car and then still have the car pass a blue slip test.

Is this just businesses justifying the reason for custom rims and overlooking it. Or the fact they just don't car.

What rims?

The NSW laws (and I think all states & territories) allow for different wheels to be put on as long as the size of alternate wheel & tyre are within certain ranges from the factory options.
If after market wheels are on the car, then the blue slip inspection should have measured or inspected the wheels & tyres to make sure they are within the guidelines.

The only thing that might have with "businesses" is that the government recognises that people like to modify cars and the $ spent on this keeps some families (business owners & staff) with an income and food on the table.

Load rating is more important than wheel/tyre size, in the long run.