Its a very interesting argument, and there are good points and bad points from both camps.
Wearing the Apple Hat:
It costs time and money to develop and provide the App store and App distribution network. Its entirely reasonable to expect that paid apps help pay for this via a portion of their purchase price.
To use the Brick and Mortor storefront analogy into play: Apple is the shopping mall. if you want to run a store in the shopping mall, and use real-estate, you have to pay for that. In return, the Shopping mall takes care of the hydro of the building, access to and from, parking, etc, etc.
Wearing Spotify hat:
After initial App download, Apple isn't involved in the Spotify application or delivery of streaming music. Why should they continue to require 30% of each and every months subscription fee? Apple is not involved in the delivery of content. Its not their network or bandwitdh and its no longer using the App store. Why should Spotify be required to continued to pay 30% of every single user's months subscriptions? Especially since with 30% taken off the top by Apple, There is absolutely no way of staying competitive in the streaming industry when Apple's own music streaming service isn't subjected to a 30% and can afford the 9.99 pricepoint