Spotify complained that apples uses its market power as the app-gatekeeper to promote its own services over Spotify's. Afaik, Spotify needs to pay that 30% royalty fee to apple whenever someone signs up to Spotify via the app-store, whereas apple obviously does not have pay itself that fee for its music streaming service.
This is the complaint: https://ec.europa.eu/commission/presscorner/detail/en/ip_20_...
Spotify has illustrated that they have no qualms themselves in abusing their market power to subvert the fairness/freeness of markets - they're currently pouring massive amounts of resources into an attempt to force themselves into the role of podcasting-gatekeeper. Hope that the EU looks into their own actions as well.