> why should Apple be forced to allow sideloading?
Because it is too large in terms of market share. In an ideal world, there would be so many alternatives, that I could just choose whatever ecosystem I want. However, we can't really reach that state of affairs easily. Without interference, the gap between the phones that support software freedom and those which do not will increase.
The "Apple should be allowed to set whatever terms" argument sounds like this: "We shouldn't impose any food safety or agricultural standards. If you care, you can just grow your own crops and cook your own food"
I think the argument I'd like to make is that when there is a oligopsony, regulators should intervene to add more competitors to the sell side. In other words, fund competitors, instead of using law to directly twist the arms of companies.
EU is already on a back footing. We are too reliant on American technology, and we don't have any players in the smartphone market. If consumers want something we should build it.
From the perspective of software makers, to fight oligopsony: https://en.wikipedia.org/wiki/Oligopsony