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.
Is it marketed as "intended for use in the diagnosis of disease or other conditions, or in the cure, mitigation, treatment, or prevention of disease, in man or other animals"? That's one of three things that gets you categorized under the FDA regulation. There's even a term for it: "SaMD".
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!)
Ironically if they stopped this gimmick they keep up to be able to charge 30% they could have the app and the developer would be the liable one. But the developer doesn't even have to make these claims, they can just put it up and say "this may detect if you're on your back".
Apple isn't liable for you looking up medical misinformation that kills you using your iPhone on the Safari browser. Why would it be for an app that is clearly made by someone else? The difference is only because of the vetting, and they only do vetting to charge their fee, otherwise they'd vet other things too.
It's incredible how companies bend backwards to bullet proof any liability whatsoever on anything, except if they can make money on it.
Now we're into a semantics meaning over "garbage and scams", as I believe pretty much all social media apps are a form of "garbage and scams". They just happen to have enough features that people like so they accept it.
Which is why Apple's page that I linked is quite a lot longer than my comment. It is more carefully worded. It also isn't based on your particular preferences or tastes, but the tastes and preferences of their target market(s), which may vary well not include you.
I am no lawyer, but I guess any such law has to take into account physical possibility. Like, e.g. youtube itself can’t possibly flag every copyrighted content streamed live/etc.