Sounds like a perfect scheme to get Wal-Mart customers to work for them for free, and like it. First you don't hire enough checkers to do the job properly, then you make pitching in your own time, effort and hardware to avoid the understaffed checkout lines look like a customer service. Brilliant.
It's really just a variation of the Sam's Club approach. That's probably where it came from. At the longer lines at Sam's Club they have a roving employee with a scanner who scans your items while you're waiting in line. When you get to the register your purchase is already there and it just needs to finalize. They've just eliminated the rover with the scanner and made that the task of the customer. You're right, they're getting the customer to do the work.
I wish this was available to us here so much. Our WalMart has 25 checkout lanes but only four to six open at any one time. The lines get massive. Sadly, our WalMart doesn't have a self-checkout. Others in the region do but not ours.
I use the self-checkouts at other stores whenever I can but I get very annoyed when the "monitor" is bored and really tries to get you to let her check you out at the monitoring station. I try very hard to get to a self-checkout register and get one item scanned before they can say anything. If someone is at a self-checkout versus a cashier lane, why do they pester you to let them do it for you? It's obvious that you WANT to do it yourself. I can certainly get in and out much faster through self-checkout.
I find it interesting that people run into self-checkout oversize issues and the 'bagging area.' Every system I've used knows when something won't fit and TELLS you not to try to place it in the bagging area but to return it to the cart. WalMart is the worldwide master of logistics. They know every ounce and square inch that an item takes up. They move massive amounts of merchandise around the world without an inch to spare. Their system would be smart enough to tell you to not put the 40" LCD monitor in a bag.