Walmart can get sued by (for example) me if took one of my prints in to have them copy it, and they did it. Maybe they can sue you on some civil thing having to do with the fact that they got sued - I wouldn't try to mess with a company that big who has lawyers on their staff (heck, they've even got meterologists on staff, seriously!). Everyone is so worried about getting sued that it's a big "CYA" game. If it's your shot then you're doing nothing wrong and they're doing nothing wrong. Think of it as a compliment that they think it's a pro-quality shot.
I'd suggest using a place like Mpix though - if I hired a photographer, I really wouldn't be too impressed with seeing the name "Walmart" on the back of the prints. The quality should be very good though, I have prints made sometimes at another store that uses the same machinery. It's not the front of the print that looks any different; it's the back. Prints from Mpix say "Kodak Professional" on the back. Which would you rather give to a client?
True story - I needed a print made quickly to send to a client, and I knew their regular photographer used a Sony Print Station for proofing. Kinko's here in town has one, so I made one at their self-serve kiosk. I had to wait for a new bride and her mother who were copying a bunch of wedding pictures - with the help of a Kinko's employee! In all fairness, I don't know the circumstances but I tried to be as nosy as I could without getting involved. The shots were, umm, probably not professional - or at least from a full time photographer who would be in business to sell prints. Didn't have a gold foil stamp or anything, but they did look like posed shots...