The fun thing about this service is that the copyright holders don't get a choice. Their content will be on the platform regardless of what they want so they have an incentive to join the profit sharing in order to better control their metadata on the service and provide better support for their users. Or they can sit and complain and lose out on any of the profits.

The real question is why there isn't a Aereo-like service where users time-share a pool of streaming subscriptions on different streaming services.