Earlier quoted context omitted.
Let's say you subscribe to something as part of a 3rd party bundle, like say you sign up for newsletter A and there's a whole bunch of other options and you leave newsletter B selected to also subscribe to that. Then the workflow for unsubscribing from B would be going through the subscription workflow for A. This would be bad for all parties: it may be difficult or even impossible for the B group to change A's subsc…
I'm gonna need one concrete example of this because I cannot for the life of me think of a single instance where bundled subscriptions, "buying Showtime through your cable provider" shouldn't be required to allow cancellation through the place you bought it. Sure, it sucks day one that 3rd party sellers don't have cancellation flows but it's not an intractable problem.
For example, I recently purchased a new car. I had to go there in person and do a whole bunch of paperwork. While I was there, I registered my new vehicle, got an insurance plan for it, and financing for the auto loan. Imagine if to change insurance providers, I had to go back to the dealership and spend 3 hours doing paperwork because that was the workflow by which I just happened to get my last provider. It would be absurd, and I'd probably never go through the trouble even if my insurance provider was more expensive than competitors.