After some decades of casual observation of people and dogs, most pet-dog people don't train their dogs beyond a call at feeding time.
A public park is suppposed to be safe for all people of all ages, all the time. A public park is not a doggie-poop repository nor a place for uncontrolled dogs.
Ergo, leash laws. Dog owners make them necessary, not the politicians. Not the children. Not the doddering Old Farts.
An amazing number of people work very hard at avoiding the fact that dogs are predators. "Not my Phaideaux!" Sorry, those fangs aren't there for decoration.
Don't want a leash on your dog? Keep a lawyer on retainer, then.
Art
I agree 100%. And, unfortunately, many of the few that do train their dogs don't understand why people are afraid or react cautiously to their fine, trained animal. If I'm walking down a park trail and suddenly encounter a friendly trained circus bear, that looks just like a wild bear, I'm going to SMP just the same.