I think perhaps the definition of a "platform", or perhaps even organized markets like a stock exchange, is a limitation/mitering/constraint/curation of choice, if that is the right term? Versus a complete free for all. The question is how much choice?

I think a tentpole of Apple is that the billing should all flow through one source, clean and uncomplicated, along with a set guidelines of program coding/behavior. It happens to make Apple the gatekeeper, and there are deeper questions thereof, but that is the constraint inherent in the App Store.

i.e. how civil rights/discrimination law identifies how all laws are discriminatory, but which categories (i.e. race, religion, sex) are "protected" categories from discrimination/differentiation.