Sleeping on your back isn’t a medical treatment. It’s a snoring reduction. You can buy snore guards, nose strips, special pillows etc to prevent snoring and none of those require FDA approval because snoring isn’t a medical condition.
It seems less likely to me that this is Apple being ruthless or pigheaded and more than you really do have to be careful how you market your iPhone apps.
I take your point; the FDA doesn't recognize "snoring" as a medical condition (but OSA is!)