Why? If Spotify provided a free service then Apple wouldn't charge - they don't charge for free apps. However Spotify want to put their product on their competitors device and go up against them and don't want to pay for it. Why is that anti-competitive? Apple isn't saying Spotify can't be on the store, or their apps can't run on their devices, or that they can't charge, just that if they do they will take a cut. It's not freezing them out at all, that would be saying "Your app is not welcome on our store".
I really don't understand how people don't get it, if you owned a business and a direct competitor wanted you to not only promote their business but cost you money through this promotion (I'm pretty sure hosting an app has a cost implication) you wouldn't be keen to say "oh sure yeah have the business, I'm happy for my customers to switch to you!".
Apple will probably lose because they are crying so loudly and the legal system love an underdog, but I really don't see what Apple is doing wrong.
Trust me of Apple blocked Spotify from iOS or didn't let them sell the apps then I would be saying it's disgusting and they should be stopped, but I'm pretty sure that's not what's happening. They are stopping Spotify from taking away the business they give Apple, and Spotify don't like it.
If people think Spotify are right then they must also agree that when you launch the Spotify app there is an advert for the Apple Music app that you can click to access.