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.