People take them home, a cashier once told me.
I tried to imagine what they wanted them for. To carry garden tools around the yard? To store yarn for knitting? To hold cleaning supplies and tote them for chores?
The other day, another cashier expanded on their uses. It seems some folks fill the basket with groceries and then walk out the door without taking the trouble to go through the checkout lane.
Ah. So it turns out that what is a convenience for shoppers with short grocery lists also is handy for thieves.