I used to be a manager at walmart, bottom line is you don't own the card, you are in effect just looking after it for walmart so that you may take advantage of the benefits it affords you. They can and will take it off you or any other employee for any reason they see fit and they are in no way required to ever give it back to you.
As I remember, the telxons log out of the associates account after been left unused for a lil bit, and unless you know the system, finding some of the things in the menus can be a pain so for rice changing, I'd say their usage is limited anything past one or 2 items, and you can't discount something THAT much without it setting off alarms in the system anyway. But people do lose them all the time, and have them stolen and they ARE very expensive to have to keep replacing.
Unless the rules have changed since I was there, you HAD to keep it with you a all times, no leaving it on a shelf while you went down the way to work, or even to the next aisle, but I know from my own expeience walking round other walmarts just how many you find laying around with no associate even nearby, neer mind next aisle.
back onto topic, the legality is it belongs to them, you have no rights of ownership over the discount card. You have expectations to not lose, damage or misuse the card, but those are on you not walmart.
yea it's shitty, but instead of riling at walmart for basically trying not to lose more money, like someone else said, find the people who caused this to be instituted by their lack of care over store property