[dual_dingo]

Well, no. This argument might be correct if this policy wouldn't very strongly incentive people to use (possibly paid!) iCloud instead and if Apple would just allow any app onto the app store (or effortless sideloading like on Android). Instead, they heavily scrutinize everything that gets submitted. They could just have special permissions for apps like Nextcloud that would only be enabled if the app behaves correctly regarding this background sync functionality.

[raverbashing]

Yes, there is an anti-competitiveness aspect here, and I agree with you that the access to the api should be allowed (given some limits)

But nobody wants to sysadmin their phone with rare exceptions