Its an interesting conundrum ;) I thought paper bags might represent a better option too, but the answer to that is "not necessarily"
From WRAP (Govt. funded charity on wrapping materials for the retail and consumer industry)
http://www.wrap.org.uk/content/carrier-bags-material-matters-0
I think, secretly, they wish to encourage all to carry our shopping home in our pockets, pants legs and arms ;)
I now have around 4 re-usable carrier bags permanently stored in the car, which is great if I happen to be shopping when I am in the car. Otherwise, its donation time at the till.