to think retailers have some responsibity?

I`ve just been out to M&S and was planning to buy 2 jumpers. Cost approx £70. I was pretty taken aback to be asked if I`d like to buy a bag to take them home in. While I`m quite accustomed to taking my own shopping bags when buying food I really didn't expect to have to take a reused ex food bag to put my clean, new purchases in. I queried it and was told "its the law". There is no obligation for retailers to hand the proceeds to charity they can simply keep it as extra profit. Am I daft to think that if I`ve spent such an amount in their shop they cant pay for the bag? If I spent £1000 on a wedding outfit would I still have to pay for a bag/box? I know about the environment but have retailers no responsibility to clear up any bags of theirs blowing around? After all they are free advertising for them. Maybe I should have asked for a plain bag so I`m not advertising their shop.
Just checked and mail order packages are exempt. Hhhmmm there`s a thought! Rant over