I know in theory that sounds "bad" but it's their service I guess? In the real world, centralised services seem to be the norm, eg. the postal service. They don't let random third parties take the mail and also mandate that you use their postage stamps to use their network, and only accept mail at their post boxes and mail offices. They don't let people inject mail into the vans along their postal routes, and don't forward mail that is from another delivery company, eg. DPD, DHL, FedEx.

I am not sure how else it'd work?? Surely it'd be like expecting the postal system to deliver FedEx's parcels, whilst not paying the postal system anything at all. That's unfeasible and unsustainable.