The fee is to unlock something on their own back end, triggered by using a non Apple device. So Apple’s license probably doesn’t apply (the corner case that could cause it to apply is the app generating a key and sending it to the Withings infrastructure, I suppose).

Nordictrack does this with iFit as well. Can’t sync to Apple Health without a paid account. Syncing is somewhat janky too.

But they are monetizing the export button you press before the api call is made.