I see your point, but how many people go to the grocery store and spend less than $12, or whatever the number is. 1 in 100 maybe? And how many spend $3 or less? Not very many...It sounds inconvenient to me.
I live within walking distance of the grocery store that my wife and I frequent, so admittedly, there are a lot of times that I will pick up only a few items on a visit. And there are times when I'll make a last minute run for something that we forgot to pick up for dinner. So I'm not sure I'm fond of the idea of having a bunch of $10 cover charges lingering on my card, and then having to watch if I get $7 back on the $3 item I picked up. With as frequently as I sometimes go to the grocery, for only an item or two, I could have 2 or 3 pending charges, waiting for the final refund.
It's a numbers game. The bottom line is, it is a cost savings experiment for the store. I am convinced there is a ton of benefit for the store and perhaps for the customer.
Just my 2 cents...