One thing you have to realize about Wal-Mart is that many of the guns and ammo sales are what are called "loss leaders". A loss leader is an item that is sold under or at market cost. That means they actually lose money every time to sell something like a cheap brick of .22lr. That's why the can sell it for so cheap.
The reason behind selling something for below market cost is to draw people into the store, where they will almost undoubtedly buy and spend more on items that the do make a profit on. The most famous of the Wal-Mart loss leaders are their $5 dvds. But they do it with milk, bread, batteries and lots of other items too. Small things that would cost you more if you were to buy them from a place like Longs, you go to Wal-Mart because you know it's cheaper there, and while you're there you end up doing most of your shopping. Its classic "it takes money to make money".
So Wal-Mart doesn't have THAT much incentive to start stocking ammo. The main office probably assume firearm culture in Hawaii i smaller than other parts of the mainland, they probably don't think it would be worth the investment. You have to also take into consideration all the money that would need to be spent on modifying and expanding the sporting goods section to house a firearms counter, and a proper safe in the back to hold the extra stock. The sporting goods section of Wal-Mart has already shrunk considerably in the last 10 years or so in Hawaii.