How many users as a percentage do you think are signing up for IOS services , discarding their IOS devices and then don't have access to a Mac or Windows PC ?

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.

This also applies if you sign up for, say, HBO from an iOS device. You can keep using the same subscription on Android but can't cancel.

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.

Good point. I stand corrected. I still don't think Apple is doing this intentionally, but rather as a side effect of it's ecosystem. Thanks for your insight