re: monopsony -- You didn't read my comment carefully. Teachers obviously aren't chained to a city, but not all of us are 20 years old and eager to hop around the country. Once you do things like buy a house / apartment, or have an SO with a job, or kids in school, or a group of friends, or family, or get divorced and are forbidden by court order from moving your child around, or have an underwater mortgage, or just plain like where you live, you stop wanting to move. Having one dominant and maybe one or two small alternate (and who knows what their labor demand is) local employers would discourage me or I'd think most reasonable people from wanting a career in a given field. Employment at will is a fine theory when there aren't monopoly / monopsony effects and both buyers and sellers have little leverage and are price takers. Remove that and people naturally see protection.