Actually, I don't think the OP was looking for legal advice. Thanks for everyone's input, though!
Anyway, on to the OP's question. It is possible that Wal-Mart is using CTCSS or DCS on their radios. This is a method to reduce unwanted interference. You can read more about it on the wiki
here and
here. If this is the case, you will need to figure out what CTCSS tone or DCS code they are using. Some radios have a "tone scan" or similarly named function that will try to figure out what tone is being used on the frequency. Check your radio's manual (or do some googling) to determine if your radio has such a function. If it doesn't, your only other choice is to try every tone and DCS code to see if one of them works.
For what it's worth, the Wal-Mart stores around me don't use CTCSS. But, your store may do things differently, especially if they are in a crowded area where there might be a lot of interference.