Did I Really Need A Bag?

Last night I got home and we realized we were out of dog food, so I ran to the store to grab a can.  It's the only thing I purchased.

As I'm checking out I tell the lady politely that I don't need a bag, but she insists.  Why?  I clearly didn't want the bag and didn't need the bag for 1 can of dog food.  So now I'm left to throw the bag away when I get home.  No it's not a big job, but why?  It's not going to advertise for the store because I'll be home in 3 minutes and no one will see it except me.  I don't need it to carry the can of dog food home, it's just as easy to grab the can and go.  Plus now it ends up in a landfill.  It also seems that when I do go the grocery store, sometimes I get almost as many bags as items.  I can put 8 things in one of those little baskets and still get 5 or 6 bags.

Look, I understand that there are some people that want all these bags, but what about those of us that don't? Can we cut back a little bit?  Especially when we ask?