Illegal jaunt

This is highly illegal in Seattle, but no one in law enforcement has ever been able to tell me why. My last dog and I used to do this - a perfectly voice-command trained Rat Terrier named Jasper - and I got a series of tickets for doing it. When asked at the time of ticketing why it's illegal... no valid response. ("It's the law, sir" doesn't count. I want to know why it's the law.) I got the $125 ticket, the $250 ticket, and was on my way to the $500.

Anyway... happy dog.