Which, despite being a nice idea in theory, now means that when you've forgotten (usually) and still need food, now you end up taking a car back with your paper-bagged groceries.

I don't understand what you're saying. I don't own a car. I live in Santa Cruz, where disposable plastic bags are also banned. I do my grocery shopping using reusable bags either on a bike or taking the bus. Where do "forgetting," cars, and paper bags enter into the equation?