XD
I know why they say that, it's policy in some places because they're trying to encourage customers to reuse bags. Asking whether you want one is meant to gently remind you that they'd prefer you had your own. Environmentalism and all that (because the squillions of tonnes of plastic used for just about every food packaging ever is okay, but those flimsy carriers where they manage to stretch out about 0.0001g of plastic are the root of all evil, apparently)
They've got the government behind them here as they're going to be forced to charge for bags soon..
I keep meaning to be all environmental and bring my own bags, but it's super hard. Who remembers to dig out some old bags and stuff them in their pockets before they go shopping.
If they charged something more substantial like £1/bag, we'd very quickly learn. But as the charge is going to be something like 2p, it's pretty obvious it's just a tax / government fund raising thing.