Is there an API to cancel an auto-renewable subscription?
I don't currently use any apps that have an in app subscription, so I am not sure what the UI looks like in iTunes when do an in app renewable subscription.
I assumed there was an api, given a receipt id or original transaction id, to cancel the auto-renewable subscription from within the application. It doesn't look like this exists.
Am I missing something or do users cancel auto-renewable subscription through their iTunes account settings? This just isn't the developer's responsibility?
There is no API for this.
The only way to cancel a auto-renewable subscription is from Settings > Store > View my account > Manage my subscription on you iPhone.
Update (as this is fairly old by now): There is no API to cancel an auto-renewable subscription. Apple's documentation suggests that the app links to:
"Opening this URL launches iTunes or iTunes Store, and then displays the Manage Subscription page."
Recently Apple introduced their new service called "Status Update Notifications" which in essence is simple webhook that's being called when certain events occur on Apple servers. One of these events can be "CANCEL" when Apple's customer support cancels user's subscription. You setup a webhook and they send you a notification with details about this event. More information can be found here: https://help.apple.com/itunes-connect/developer/#/dev0067a330b
I added this reply because this is what Apple recently introduced.