The private company argument has limits. Common carrier status for one. At least in the EU there also are exemptions for repairing and interoperability.

In my opinion repairability should extend to the ability for the owner of a device to side-load any software.

If apple wants to police their own app-store, fine. But they shouldn't be allowed to exclude installation through other means. If they had not complete control over the ecosystem then they would not be in the situation where they can and have to play deputy sheriff for the government.