Well, being the former OReilly boy that I am, I know that we had to start carrying a second/cheaper line of parts when we moved into an AZ market or they moved into one of ours. This to compete with them on price...and quality (or lack thereof), then once the customer was in the door, we'd try to "sell them up" to the better line of parts that we had always carried.
Put another way, the typical OReilly and NAPA sees about 50% of their sales go out the back door being delivered to dealers and mechanics. The typical AZ is closer to 25% (unless they're the only game in town) because mechanics won't put that junk in their customers' cars and deal with all the warranty claims.
Carquest used to have a good line of parts too...not sure anymore since they've been bought out by Advance.