I can't work it out, as I would have though, things that make your car stand out, make it less likely to be stolen, as it is easier for the police to spot.
The only thing I can guess at, is, if the car has a panel damaged, and the sticker is over two, fixing it is more difficult/costly. If the paint has faded, you can't just remove the sticker from the good panel as it will show, and trying to get a replacement and match it up could be very difficult, so they would then have spray two panels, with the knock on effect of more stickers across more panels.
Having said that, I am making excuses, as all they have to say is replacement of any stickers, or damaged caused by the stickers is not covered.
The whole thing is ridiculous now though, as you have to declare anything that is non-standard, and how are you supposed to know on a 15 year old car what it did and did not leave the factory with?
When I had my Vitara, there was also a model called a Big Foot, which had Bullbar, Spot lights, Wide Wheels, side steps, Dummy roll bar, Stickers, Thatcham approved alarm/immobiliser etc. etc. I later through my non-related job, found out that all these were fitted by the importer, at the docks in Sheerness, not the factory in Japan, but When you purchased it, it was implied it was a factory limited edition.