The argument seems to be that plastic bags are bad for generating litter. Also there will be a general feel-good argument that plastic is bad so banning bags is

The argument seems to be that plastic bags are bad for generating litter. Also there will be a general feel-good argument that plastic is bad so banning bags is good. It's good to feel good as the planet fails to deal with serious environmental issues like global warming. Stuff like this lets politicians court the green vote with measures that are relatively deliverable, and they probably sincerely feel it is making a difference.