I think any system of rules is going to look weird and organic when they have millions of people trying to rule lawyer the system.

Like the basic tenant of “we take a 30% cut of the sale of all digital goods on our platform” is pretty straightforward, you only run into weird rules when you try find ways around it.

How are we trying to find way around it? We don't want to sell anything via IAP because it does not make sense in our app. But somehow we are forced to do it.

Look, I don't at all think you're trying to get away with something -- I think you're caught up in weird rule that was intended to plug the loophole of "selling a stick of gum that comes with a free bottle of water." From the reviewer's perspective they see that when personal trainers sell their services to clients they're also selling your/their white-label app. To Apple that's a sale of digital goods and they want their pound of flesh. The fact that it's bundled with an IRL service doesn't seem to matter to them.

You might be able to get away with skirting this rule if your white-label app is just a client to a fitness tracking service. Then it should fall under the same rules as Netflix and other "reader" apps.

tenant (occupant or renter) -> tenet (principle or belief)