Considering that I cannot ever remember any reusable grocery bag except the one that folds up small enough that I can I keep it on my person at all times, I have a hard time criticizing anyone for not remembering theirs. And the trend I've observed at the checkout line is that those with less (apparent) resources are more likely to not have bags with them, so I have an even harder time criticizing them. I have no doubt that there are a lot of reasons I have no way of seeing why reusable bags might be hard for someone.
Right! I live in a city that first banned plastic bags and now taxes them (you have to pay 7 cents per bag if you forget your reusables). All the propaganda about reusable bags says, "keep them in your car!" Um, OK. I don't have a car. Lots of poor people in this city don't have cars. It takes a concerted effort to remember to have a bag on me at all times; we've had to try tricks like hanging them from the front door knob so we see them when we're leaving, or leaving some in our granny cart that we take to the grocery store. And we STILL end up turning back after walking a few blocks sometimes when we realize we forgot them.
It also means that if I'm out and about, or taking a walk, and I realize I'm right near the grocery store or CVS or whatever and really should pick up the ____ I'm out of while I'm here, I likely don't have a bag with me. Because I don't carry one every time I leave the house unless I'm specifically headed out to buy something, though I guess I should? This makes me less efficient.
My main complaint about the bag tax/ban is, what are we supposed to put the kitty litter in now? I always scooped it into a plastic grocery bag and took it out to the alley so the house garbage can didn't smell like cat pee. It's lucky that I work just over the city border where bags are not taxed or illegal, so now I stock up when I run errands at lunchtime.