Plastic grocery bags can be a real scourge. They are made with petroleum, which we KIND OF need for other things (like transportation). Their creation involves tons of toxic chemical byproducts. They are so flimsy that you end up getting 20 of them per grocery trip, since just about everything has to be double-bagged.
They exemplify our "single use" culture, since you're lucky to get one more use out of them - at least half of them split before you can finish unpacking your groceries! They take up landfill space, recycle poorly, and have an unfortunate tendency to become airborne, flying into our trees, fences, and into our oceans, where they destroy marine life.
As you might expect, there is a lot of griping about this in Seattle. To which I can only say: grow up! It is not that big a deal. And after the first few times, you will certainly remember to bring your reusable totes. The only reason people forget to bring them is that they CAN forget. You don't forget to bring your wallet or your car keys into the grocery store, and soon you won't forget your tote bags, either.