I wonder if it works at all. I doubt it has any success in most of the US.
Could it be they got sick of being confused with Mormons while going door to door so they had to come up with something else to make them stand out? Or maybe it is an effort to increase persecution by way of being made fun of for being weird. Or is just that they think that people just seeing the sign is a good enough witness so Jehovah can kill the wicked without compunction?