If a city only allowed one retailer, that retailer would be under a scrutiny for their excessive margins. Because of the competition among retailers, each retailer can set their margins. The argument that residents could move to another town if they didn’t like to have only one unregulated retailer doesn’t hold water. This is undue hardship on the consumer.
Developers decide how much their apps or IAPs cost at the App Store. They can freely set their margins.