The reusable bag quandary

Ok I do my part, small as it might be, to help the environment. For the past few months I have been using reusable shopping bags. You see them in every store, the cost a buck or two. So why is it every time I go into a store and place my bags on the counter, without fail, the clerk asks me "Do you want everything in here?"
No, I just decided to put my shopping bag there so you could look at it, and ponder how nice it is!