It wasn't a "nuance". It was about what was legal and what was not. Was it ok for them to think that all VA driver's licenses with a po box only were illegal? It definitely was not a nuance to them. It would have caused issues with every customer that came in with that particular issue. Now they know.
I highly doubt you were the first person ever in that walmart to try to use a DL with a P.O.B and told that was unsatisfactory. It's not illegal or even wrong, it's a business applying practices that allow them to give minimal training to employees while steering clear of legal issues.