Two things:
Wouldn't every single item in the store need to be connected to the internet to verify the transaction? I know internet of things is getting popular, but that would be one hell of a cost. The follow-up thought I have is well, instead of attaching the internet to the device, put it in a container that only opens once the transaction goes through. And then I realize that I've brainstormed up a vending machine. So my question is: how come vending machines aren't used for more items? The interaction is essentially the same to what you're describing: you walk in, interact with a thing, receive product, and walk off.
Second thing: you wouldn't be able to get rid of cash registers until the vast majority of customers were using bitcoin. Yes, the cost of the system might be offset by not needing cashiers/cash registers, but it wouldn't offset the cost of losing 95% (on the low end) of your customers.
tl;dr Many negative things. Sorry
I'd also like to point out that part of why I structured the OP as I did was to provide my honest feedback as a consumer about what I would like my in-store experience to be. I like to get in, not be bothered or held up by anything, get what I need, and get out. So, saying "many negative things" is irrelevant. It's more a comment about what's seemingly inconvenient about the tech, not about the actual experience (which is what matters to people). Of course, maybe most other people prefer some different type of in-store experience, but no tech will ever succeed if it doesn't do what it should be doing, which is making life easier for people in ways that they want it to be.