I'll give them two things: 1 - using fewer plastic bags is probably a lot more beneficial to the environment than emitting less CO2. 2 - they didn't jump right to banning the bags - nor should they jump there eventually.
But honestly, I'm not sure why the government feels like they have to do anything about this. My Zehrs recently started selling "green" shopping bags for a buck each and I bought one without a government program. So did a lot of other people. The damned things are selling like hotcakes, government intervention-free.
... it's not a landfill shortage that drives people to want to reduce plastic bags, St. Godard said. "The biggest issue from a public standpoint is the problem plastic bags cause with litter. People see them in their communities, they see them on the way to work, they see them at the park," she said.
But hell, why not use the government to take yet another community-driven incentive out of good behaviour. Sheesh.
UPDATE: Tim Hudak loses points. (h/t: Freeway to Serfdom)