My cynical take has always been that this is their justification for charging a subscription fee for features that shouldn't require one (e.g. an alarm clock or the ability to change the temperature on a schedule).
There are a couple of open-source projects around jailbreaking it:
- Probably the original attempt: https://github.com/bobobo1618/ninesleep
- The much more polished and usable application built from that starting point: https://github.com/throwaway31265/free-sleep
The latter is even able to do the signal processing needed for heart rate on-device.