Those stores are violating FEDERAL LAW.
Doing so is a great way to get a visit from a BATFE agent. The clerk that sells ammo to underage people is the one that will take the hit. At a minimum it's going to cost them several thousand dollars, either in fines or shyster fees.
Doing so is also food for those that would want us to return to having to log ammo (handgun and rimfire at the time) ammo purchases in a bound book AFTER seeing ID.
Walmart and Atwood's are following Federal Law and keeping their clerks and the store out of trouble.