" There's no public right to have a music app be charged a certain amount that's called "fair". Spotify could charge nothing to consumers, and be charged nothing by Apple. It's their choice. It's Apple's choice."
Competition laws place limitations on what a business can do. They're not free to sell or price entirely as they please.
In this case Spotify are arguing that Apple is using its dominance of the smartphone market to give an unfair advantage to its own music store. Whether or not this is true is up to the courts to decide, but it's not a frivilous case; there's a real risk to Apple that they lose.