A lot of major tire stored do this...not just walmart...I see also where they will not install a different speed rating tire then what the sticker says on the door...Just want to add (didn't see it mentioned yet) that Walmart is an absolute stickler about not installing a different size tire than what the door sticker (glovebox, wherever) specifies. Doesn't matter if you already have a different size and replace with equal specs. They'll outright refuse.
Only exception I've encountered was on my 72 Pinto wagon about 10 years ago. They couldn't really do anything with the bias-ply tire sizes, so they agreed to install what I had purchased. I had done all the math, so the new tires were as close to exact as humanly possible, but they didn't know how to verify. Lol.
OT, but I really miss that little car. 2.0 auto, but really fairly peppy if you wound it up and shifted manually. Rebuilt the head with a performance valve seat job and new valves and springs, used a fairly hot cam ordered direct from the UK, a brand new Weber carb. Would keep up with traffic on the interstate to about 75 mph (where the 3 apd auto pretty much topped out). Daily drove it 40+ miles each way. Anyway... lol
There may be a company-wide policy at Walmart that waives the "must match sticker" rule for anything old enough to have originally come equipped with bias plies, idk.