Because in many places soliciting is either not allowed, or requires businesses or charities to go through regulatory bodies before being allowed to go door to door.

Further, criminals have hidden under the guise of salesmen, charities, etc to case whether or not people are home, a home has security systems/dogs, etc.

On my street our neighbors (hint we have black, white, asian, and hispanic neighbors) call the non-emergency number every-time their is a solicitor to check if they are allowed to be there because a few years ago someone from out of town was caught burglarizing a nearby home after pretending to be some sort of door to door salesmen. I know this because a Sgt at the local police department lives two doors down from me.

And what do you know? We no longer get bible thumpers, landscapers leaving bags of gravel with a business card, security system salesmen, or kids selling candy for "charity" anymore. The fact that you automatically think everyone who called the police is racist just shows you have an extremely sheltered, nonsensical view of the world.