You’d have thought that compulsory desexing and microchipping would be in place through federal law in the USA. This is because of the intractable problem of some cat owners letting their female cats have one litter before spaying leading to unwanted cats. Compulsory microchipping would enable the authorities to keep track on who is informally breeding cats. The problem is enforcement.
You could even go as far as compulsory registration of cats but that would be very unpopular. It would infringe upon a citizen’s freedoms many people would argue.
Anyway if it is impractical to have a law which makes desexing compulsory, it would be impossible to have compulsory registration. One Greens councillor in Australia (Gabba ward of Brisbane) wants compulsory registration of cats and the price for registration to be artificially high to put people off adopting cats. The objective: to reduce feral cat numbers. Brisbane is in the state of Queensland, Australia, where they have removed mandatory registration from the statute books but all cats and dogs in the state must be microchipped before sale. There is an argument to make desexing and microchipping mandatory and done simultaneously.
The difficulties in enforcing certain laws prevents them being enacted. In Australia, mandatory desexing was considered for the Animal Management (Cats and Dogs) Act 2008 but it was not included because of the impracticality of enforcing the law. The politicians could see a law making spaying and neutering mandatory becoming ineffective, a waste of time, and therefore they gave up on it.
The 64,000 dollar question is how are local or federal governments going to try and reduce the level of births of unwanted cats? There remains far too many unwanted cats at shelters and too many feral cats. How can the root causes of this situation be tackled? Education is cited but the hardcore of cat owners who allow their female cats to procreate are unwilling participants it seems to me.