See also the RFID tags Uniqlo and Decathlon use for self-checkout. You just place your shopping bag in a bin and it reads all the items within in a second.
My guess is that right now this isn't viable for lower margin food products. Uniqlo and decathlon probably don't sell that much for less than £10. It's going to be a while before you can reasonably do that for a pint of milk.
These would be 290 yen ($1.90) 790 yen ($5.20) in Japan respectively.
I still don't think an RFID will eat that much into their profit margin, but Uniqlo is in the unique position of having stricter control over their supply chain. All the products they sell are Uniqlo-branded products, manufactured specifically for Uniqlo, and they can just have RFID tags embedded in the price tags for all the products with little extra cost.
Grocery stores sells cans, plastic jugs, glass bottles, shrink-wrapped fresh produce and even plain vegetable and fruit or grain and legumes by weight. All of these comes from a wide variety of manufacturers that don't produce directly for you (or even have any kind of direct relationship with you).
The wide variety of form factors makes attaching RFID to everything unrealistic, and even for the stuff you could attach RFID to, it's not like manufacturers would do that for you.
