Excellent question.
And the answers is: propaganda.
The recycling crowd is trying to guilt-trip us into feeling environmentally guilty by falsely naming these plastic sacks. I think that there theory is that by making us feel guilty, it will cause people to buy into their coercive anti-bag agenda. (only partially sarcastic).
This is part of a much larger environmental agenda that would have everyone be vegetarian, ride bicycles, wear sweaters, and kill all cattle. (only partially sarcastic).
The fact is that most plastic bags are, in fact, single use. Only a comparative few are reused, even just to take out the trash.
All of that said, I wish the grovery would go back to paper sacks.