There are several reasons why I believe the Walmart service indicator reports that a phone will not work in a given ZIP code.
To me, it appears that the entire Decatur area has strong Sprint coverage.
However, the bottom line is that you should determine for yourself whether you will have a good chance of this phone working at the exact addresses you primarily plan to use the phone. Since this phone works on the Sprint native network, go to the Sprint coverage map and hone in on the exact addresses to determine the approximate coverage.
If you determine that you will have strong Sprint coverage where you plan to use the phone, I'd go ahead and order it and if by chance you don't have good coverage, return the phone to your nearest Walmart. You don't give up the opportunity to return the phone if it turns out you are dissatisfied.
Sprint coverage map (copy-and-paste):