Ahhhh bush lawyerism at it's best.
A yellow sticker (or defect notice) is an order to have the vehicle inspected by a qualified (supposedly) mechanic or professional because the police offcier or DOT (or whatever they call themselves now) officers believes that there MAY be a defect.
A friend of mine still in the job said to me once "we target those that draw attention to themselves". Do you really think WA coppers in general woke up one day and thought "hey I hate motorcyclists, I'm going to get them".
Or do you think in general we often bring it upon ourselves (myself included) with the way we often ride, and the modifications we do on our bikes.
With modifications there was usually a simple way coppers used to consider modifications, at least this was true back in my day.
If the vehicle doesn't come out of the factory like that then it could potentially be contrary to the vehicle standard regulations (WA) and therefore it is possible to issue the vehicle with a defect notice.
If you mod your bike, or wear the tyres to the bar (or the canvas

) then expect that you may one day run foul of a yellow sticker. Make sure you keep your stock parts just in case.