Er, OK, but that's a counterfactual, i.e. the plastic bag ban (or levy, really) is motivated by the former, not the latter. Incidentally the plastic bag levy here was campaigned for very strongly by the Daily Mail, due to its editor being upset by bags littering his picturesque English village. I thought it was a bit tokenistic myself, but it seems to have worked out OK.
I do think there are outbreaks of paternalism/maternalism(?) by liberals. But generally insofar as they do that, it's because liberals aren't really on the left or that they are too concerned with placating the right. Now when the _actual_ left gets power they do indeed seem to end up bossing people around rather a lot, but it's still not 'nannying' or 'puritanism', it's class hatred and a desperate attempt to make the theory work.