I was one of those who did commonly use Walmart for oil changes. It was convenient, and generally cheaper than most places.
As for PricklyPete's - While the job is trivial, the people usually doing it are very close to minimum wage employees who are far from mechanics.
My thought is, most of the places that do these sorts of things have the same minimum wage monkey doing the same thing, and since it is trivial, it shouldn't matter. In my few years of letting Walmart do it, the worst experience I had was they had left a new bottle of oil under the hood. Pretty easy to spot as the hood wasn't latched. Shortly thereafter, I started doing it myself. (Shortly thereafter, I got a bigger garage and the necessary equipment.)
In any case, I don't know that it really matters where you bring it to. It's a simple job, and eventually someone is going to screw something up because they're distracted with something else or in a rush. So in the end, the only person you can control is yourself. Bringing it to a dealer is going to see an oil change being done by their greenest "mechanic" who may be as likely as the Walmart mechanic to mess it up.
Personally, I might still use Walmart if it wasn't for switching to synthetics and their bogus up-charge on that. I believe it was $25 for a conventional vs. 55 for synthetic. A 5 qt jug of conventional is $15, a 5 qt jug of synthetic Mobil1 is $26.50. Since the other parts are the same, I would be paying a $30 premium for oil that cost an additional $11.50.
In the end, I enjoy wrenching, and even a simply oil job gives me the peace of mind in know that all of the things requested were actually done, and that my car wasn't just driven through the stall and handed to me on the other end without getting the oil changed.
All in all, my surprise to this story is Walmart's reluctance to pay for their mechanic's negligence. In the end, making the customer wait this long for any resolution will cost them a helluva lot more than $3000 when other customers decide that Walmart may be unwilling to resolve any dispute they may encounter in the future. They give the Weightloss - Makeover Edition participants $50k, so what the hell is $3k for a simple gesture to show your customers that you will own up to your mistakes?