I think everyone is missing the point here. Spotify does not say "we don't want to pay Apple %30 cut" since they were already paying that for a long time.
When Apple Music was introduced, Apple matched the price with Spotify. However, Apple is not paying any %30 cut and can still make more profit than Spotify which charges you the same price. What Spotify says is "if I have to pay %30 cut so that I have to make the $9.99 deal $12.99 to create profitable business where Apple can still charge its users $9.99 and make a lot of profit".
That's exactly why senator who is a presidential candidate for 2020 wants to break Apple by splitting App Store from Apple's control. Apple is now a competitor in the market but it uses its platform advantage unfairly. Of course, Apple should charge Spotify with whatever cut they want. However, Apple should not restrict Spotify from directing users to its own platform where people can get the service for $9.99. They might restrict this to many applications since they have to earn money from App Store. However, Apple hits two birds with one stone on this one which is completely unfair. They are making profit from App Store with %30 cut and restricting any directions to Spotify's own platform. So that is one bird which is fine. However, it causes Spotify to lose money if Spotify does not increase the price. Apple still charges $9.99 and hits the second bird. Do not forget Apple can send push notifications for deals. Another solution may be forcing Apple to add the %30 tax on Apple Music as well by making the price $12.99. But this is not users want.
Owning a platform and having rules do not mean that they are completely fair. Then, companies may think that "okey, the platform is mine. So I can do whatever I want".
Advocating Apple on this one causes trouble to us, THE CUSTOMERS. Competition will be lost, and therefore the users. This is basic economy principle.
I will give an example, a political one but it will clarify these. Let's say there is a governor in a district. And, there is company X in an industry. The company X accepts the district rules, taxes and all. After a while, governor sets up a company (Y) in the industry where company X is also serving for years. What do you expect from the governor? To apply all the rules and taxes on his/her company Y. However, governor is not paying any taxes for company Y. So that, company X finds itself in an unfair market. I think you will get the idea.