Mastercard and VISA provide the security hardware, banking interfacing controls, APIs, fraud detection and security, chip card and terminals. It's not wholly dissimilar.
Now sure, perhaps Apple provide the ecosystem in a broader sense, but the customer is already paying for the phone and the ecosystem. If these are payments for processing a fee - which is what they are defending, it is not dissimilar.
If they charged 5-15% it would not be such a major problem. The problem is that I have a Spotify account paid for online. Spotify developed the app and I have an account. I am blocked from using that by an ecosystem that inherently thinks $100 billion a year is a suitable surcharge revenue to manage an ecosystem that could be managed for less than $10bn
There is an inherent national interest / protectionism argument here that the EU and the US stock markets (and governments). The EU is obviously looking after its own interests in a field where American technical behemoths dominate. Apple's annual revenue is higher than the GDP of 2/3rds of EU nations.
Personally, I think it's illegally extortionate. Where say Spotify would need to pay 30% of its annual revenue from Apple users, probably in the region of $3bn per year directly to Apple.
It did not help Apple's case when Google dropped the revenue charge to 15% (albeit with a lot of nudging). At this point, I think it's just how long Apple executives can justify holding onto their model, I personally do not think Apple's ecosystem is fundamentally better than any other 'game ecosystem download' system.
For reference, Epic games provides an almost identical service and ecosystem for PC gamers taking just 12%, and I would say PC gaming and security is a lot more complicated than the Apple infrastructure, if for no other reason that it supports a vastly wider array of hardware and security considerations.