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