You assume that the only legitimate arrangement is that a piece of content can only be available on a single platform. Wouldn't we think it is weird if each book could only be sold by exactly one book seller?

What if the platforms competed on offering a better user experience or other affordances or price?

If there were some way to break the normalization of exclusive distribution, that would tilt things back in favor of the consumer, but I won't hold my breath for the legislation.