Maybe it's more about the threat potential
They don't need to be really effective - just put a bunch of nuclear
bombs and laser weapons in Earth orbit and let the rest of the world's
imaginations do the rest.
Kind of like when jewelry stores would put tarantulas in their display
windows and had their diamonds in aquariums full of pirahnas: They
counted on the generally lower knowledge level of the average
criminal and the exagerrated danger of these creatures to be
intimidated enough not to rob them.