We have the same law in effect in my city. They do not have any way to enforce it. The way they are "supposed" to get folks is when they go in to City Hall for the annual dog license. But the folks who don't spay / neuter do not get the license, either.

It does give Animal Control some "clout" if the dog escapes and is roaming, and is taken in. The only way you can get your dog back is for the spay / neuter to happen. you go set it up at your vet, and ACO takes the dog in for the appt. You pick the dog up later that day.

On the cat side of things, it is even harder, as we don't require cat licenses. Folks who leave their cat out roaming, when it is not fixed, often (in my city) can't be bothered to get the poor thing out of the shelter in the first place.

So it helps a bit. When the folks go in for a dog license and "didn't know," those dogs will be fixed.