That is what I said. It is not about upselling stuff for those who do not want it. It is about making it easier to connect with people who do. This makes sense for some situations but not for others. The grocery shop example you mentioned is great, I agree with you there. People who don't want to be contacted, should not be contacted.
I'm not saying that the way Paddle implemented it is the correct way forward. Personally, I'd rather there was an opt-in option to share data on the payment form. This way, if you want to be in contact with the seller, you can flag it and the default option is not sharing. This is simple and would allow you to remain "not a customer" while letting those who want to be in touch to have an easy path forward. This is not possible inside the App Store.
The App Store model —and Amazon and others as well— make it really hard for you to find who the customer is. That is not because they're thinking of you. They're doing that as yet another vendor lock-in mechanism to prevent the seller from leaving or doing any kind of business with that customer away from them.
As ~NhanH said in another comment in this thread, you can put a form on an app. Yes, you can jump through a lot of hops, and maybe Apple will allow it through. It is death by a thousand paper cuts. From their PoV it is their customer, not yours. From their PoV, the seller is a customer as well.