Now that I've puzzled out the thought. It's a good idea that would encourage reusable bags but sadly, cards were chosen and I don't expect anyone to change that because it would end up costing more for the companies than not changing it. I can also think of way to get your discount without having to have the entire bag
I think having some sort of government backed, push for reusable bags would be an easier way to solve the problem rather than relying on people who don't get any monetary benefit from doing so.
Don't worry, you only need to read one time at a time? /j