This is not, cannot be, and shouldn’t be math. Yes, it’s all “arbitrary”. Unhealthy impulse-items at the checkout are going to be regarded as quite unethical, by a lot of people, for really obvious reasons. The approaches you’re trying to use to “disprove” that isn’t how any of this works.
Many things are bad. Some are worse than others. Ones that are intentionally manipulative, as the impulse-buy aisle is, and greedily pushing high-margin products that are also unhealthy? Yeah, that’s an extremely shitty thing to do, no matter how common. The motivation is 100% greed, not delivering a better experience (as simply making candy and soda available in some normal aisle might). And in the Year of Our Lord 2023, every person choosing to create impulse-buy areas knows exactly what they’re doing and the effects it has.
The Starbucks bottles in the checkout aisle are, similarly, bad. The Starbucks that you have to walk over to, look at the menu with calories printed right next to each item while you choose what to buy, then stand in a second line, check out again, then wait at to get the drink, isn’t bad in the same ways. It might be bad in different ways, and to a different degree! But it’s not the same, and you’re not going to be able to construct some proof that requires I condemn those equally or else condemn neither, because that’s nonsense both in the specific terms of what we’re writing about, and also because it’s not a useful way to analyze or discuss these sorts of things in general.