The seriousness in which the MTA and CFA enforced the laws on taboo weapons frightened all of human space. Practically no one except the lawless dared to cross the two biggest organizations that collectively guarded over humanity.
It was for their own protection. During the Age of Conquest, the humans who conquered the galaxy star by star possessed no bottom line. With their awesome capital ships, they showed no remorse in bombing highly populated alien planets to glass.
Saturating a planet with thousands of nuclear missiles proved to be the quickest, easiest and dirtiest method to wipe out an entire planet's worth of aliens.
Over time, humanity began to be more creative as they sometimes wished to occupy a planet instead of turning it into a lifeless radioactive rock. They experimented with biological diseases and chemicals as well as radiation as a way to cleanse a planet without rendering it uninhabitable.