Actually Gary T hit the nail squarely on the head with this post:
Auto Zone is not licensed as an Auto Repair Dealer.
Per the BAR reading codes and offering advice falls under auto repair.
The BAR has some serious penalties for operating an unlicensed repair shop.
In addition even if a parts store were to get licensed there are a whole raft of additional rules that they would have to conform to to read codes.
Bottom line it’s easier just to not offer that service.