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.