I don't think this law is crazy at all; I think it is fair to the consumer and to the store. It's not a huge amount but enough of an incentive for the store to do things right. My time is valuable, and I work hard for my money and I'm not giving either time or money away freely to any business!
Probably 50%-75% of each grocery trip is strictly sale items. I find mistakes ever so often and they are never in my favor. If I catch it at the checkout, I say something and that's fine. If it's a small error of a few cents it isn't worth my effort. If it's more than a few dollars, than you bet I'm going back and I'm getting the bounty. I actually went shopping tonight and had a problem. I got out to my car and I examined my receipt and found 3 incorrect amounts. They actually asked me to go grab the sign for one of them. I didn't feel bad in the least for asking for the bounty since I basically had to do their work for them.
Also, they DO have to honor what is on the sign at purchase, even if it is a sale sign.