The lesson is clear — terror works
I wouldn't jump quite so far in that direction. There are many types of stickers one can put on a car that attract attention and may lead to vandalism of the car.
In this instance, they were Christian stickers and the obvious perps would be muslims in the UK, but here in the good old USA I allow no stickers at all on my car, with the exception of parking stickers which allow me to use various parking lots. I don't like these, but the people who issued them seem to have no idea that anything which makes a car stand out compromises your security and safety.
Does the insurance company ban all stickers, only Christian ones, or only "custom" ones?
* * * * * *
In my rural neck of the woods, it is common and popular to have NRA stickers, and ones which proclaim your elk or deer hunting abilities. Drive down to the SF Bay area, and a PETA person will vandalize your car without a second thought, and believe that it is the moral thing to do -- saving the animals and such.
If that PETA person drives up here with a sticker on his car a couple of rednecks are liable to have a serious discussion with him in a dark alley.