I believe in the private sector as the solution to most of our problems, but the private sector believes in the profit motive. There's nothing profitable about conservation, sacrifice, and restraint. They're the opposite of consumption, greed, and rapid growth.
Initiatives like this, cap and trade, EPA policies, etc. try to change that so that there is profit to be made in being conscious of the effect companies have on the environment. They do so because without that corporations are most likely to take the easiest route -- not giving a damn.
I really don't see how a private interest group could possibly encourage corporations to be less profitable in the name of slowing/stopping Climate Change.
Furthermore, maybe these bags won't be the end of the world if we fail to stop them, but you can apply that logic to every single tiny change we could make to positively effect the world, all of which add up to big sweeping advances when taken together. We do need to perform a cost-benefit analysis in deciding what we're going to do in order to preserve humanity's place on earth, but 5 cents extra for a bag is hardly high on the "cost scale."
If a scientist who is very knowledgeable about the issue runs the numbers and it turns out this initiative doesn't make sense in terms of gains versus losses, then I don't think this legislation was very intelligent. However, I'm not going to prejudge because I don't know. I just disagree with the general sentiment of refusing any governmental involvement in stopping climate change: it has to happen and it needs to start as soon as feasibly possible.