Why? Because houses with disposals put 20% more grease into the sewage system. That grease can lead to sewage backups. Then the city has to spend money unclogging the grease or putting chemicals in the sewage system.
People caught installing or replacing garbage disposals face fines of $25,000 PER DAY.
Appropriate use of law? Will a black market in in-sink-erators emerge? You decide. And discuss.