More like “Code Enforcement does not want to use a step-ladder to look for violations” law. Really it serves no-purpose. It is more a rule you would expect from an HOA than a municipal ordinance. It is not uncommon for there to be an ordinance regarding fencing height and material. However, such an extreme height restriction is usually reserved for front parcels. Since most places realize one of the major reasons for a fence is A. privacy and B. to keep dogs in, they tend to allow a higher fence in the back of the house.
A 3 foot fence is the city telling the property owner that, effectively, no dogs are allowed. And yes, privacy? ha!
They have drones to do that now. grrr.