This is a very good theory but I have a NRA sticker on my vehicle and Knock on wood it has never been broken into. But If I go by the theory my vehicle should be broken into on a daily basis.I also think that while showing support for the NRA and 2nd ammendment rights with stickers is great, it is also a sign for criminals to break into your car looking for guns.
There is more than one way to look at a situation! The way I look at it is with this sticker the thief is going to think twice about breaking into my vehicle because he has no idea when I walk up behind him, and I am going to be carrying and take care of the situation right then and there.
Ain't scared, have sticker!