I’m a bit confused. Do the supermarkets in America not have infant seats on them already? Or an attachment to fix a car seat securely onto it?
However, lying a baby across the child seat in the trolley is not safe.
On the other hand, it is not a realistic expectation for the mother to go shopping without the baby. I have a toddler and an infant, and there is no way I can go shopping without them, since my husband works. I do local grocery shops with them, and do my big supermarket shopping online instead. So as Ash said, maybe just offer to help out instead of criticising.