Many places won't do it for liability reasons. If they mount it they are deeming it safe. I had some used excellent tread tires kan rock wouldn't mount, and that was years ago.
That said, I try not to shop at Walmart, I needed a pocket knife today and Walmart was all that was around. I waited 10 minutes for someone to open the case, then they couldn't find the keys. I waited another 5 or 10 minutes only to be told the night clerk took the only set home.