That's not store policy, it's Federal law. They aren't allowed to sell pistol ammunition to anyone under the age of 21 years, or they're in deep trouble. You also have to realize that Wal-Mart doesn't put "gun people" in the Sporting Goods Dept. They may work that counter one day and Housewares the next. Sporting goods is a very small part of their bottom line, so it doesn't get much of their attention. We should just be glad they do sell some ammunition, even though I haven't shot factory ammunition, other than .22 rimfire, in many, many years, but if I did, I would be glad it was available.
When anti-gun people complain that guns and ammunition are available without restriction, I have to argue with them. Cars are much more dangerous, as proven by the death rate from them, but you don't have to undergo a background check to buy a car, or even have a drivers license. Anyone can buy one. You also don't have to show ID to buy gasoline, with is much more dangerous than ammunition. You don't have to undergo any training to put that gasoline into that car, either, and the two of them combined kill thousands more people than guns every year.
Hope this helps.
Fred