Oh, and before someone raises the question, let's address the 'how do you enforce it?' matter.
Simple enough. It's enforced the same way all laws are enforced - by civic responsibility.
The vast majority obey laws out of duty and respect for others without need for any surveillance or enforcement.
Let's draw an analogy with drink driving.
Most of us don't drink drive, despite the fact that we could get away with it.
However, I'm sure it is a law that is occasionally broken, but the person may have had just one pint too many and no one has reported them and they've driven home carefully. Just like that person who takes the dog for a walk, let's it off lead but no one sees it (or cares) so they get away with it. Of course it happens.
However, sometimes someone has too much, is reckless, and they have an accident and someone else may be harmed or killed, and the Police get involved. Similarly, someone lets their dog off the lead and attacks someone.
A law such as 'dogs on lead' will get broken by some, just like drink driving. But I am sure that doesn't mean we would want drink driving laws repealed.
It works the same way.