Well it does need to read individual tag barcodes, so it is indeed one at a time, still you could make an automated camera+beam device, hide at chest level, walk through an aisle as if looking for something, then pick up something in the end of the aisle and go to checkout.
I don't think anyone is paying attention anywhere near enough to pick that up. Additionally, one could read some barcodes and make quite cheap battery-powered narrow beam emitters to be placed in store aimed at particular tags that would only power up once a day at a random time.