I fully expect Apple to cancel my subscription via Mail and refund me back dated to when the letter was post dated !
My point is Apple isn't doing this on purpose to you, you're just in a very strange use case
You keep making these claims that I'm sure sound more outlandish to you, and I think all of them are quite reasonable.
The purpose of a system is what it does.
Very few people use Linux as their desktop is. To be honest they should allow you to reply to the email and allow a cancelation, but you following into an edge case isn't deliberate.
If you subscribe to HBO on Google Play, you can keep using it and cancel it on iOS.
The asymmetry is bad and Apple should be held responsible for it.