I lived in Europe for 3 years, so I know that not every store in every country provides bags when you shop--but this 'ain't' Europe.
I care about the environment. I buy reusable shopping bags but don't always remember to bring them with me to a store. I also recycle, watch our water consumption, etc. But if I am in a store (and you see no reusable bags in my hands), do not ask me if I want a bag. Of course I want a bag.
I am not talking about buying a box of cereal or pack of gum. I'm talking about at least 2 bags worth of purchases. "Would you like a bag?" No. I don't want a bag. I much prefer to carry my wet/dry vacuum cleaner filter, 8 curtain panels, food processor, gym socks and jeans in my bare hands and make several trips to my car until I retrieve it all.
I am sure they are mandated by management to ask--but where is the common sense? If you see I've got a ton of stuff, don't ask me if I want a bag.
Some places charge you for bags, so I'm sure they are required to ask before putting that extra charge on your bill. We use reusable bags as well, mostly for the commissary though. I forget them everywhere else.