The difference is that the LFS in the area have a method of QTing their fish if they are sick. Petco has a general policy about just dumping the fish in the tanks and seeing how they fair. They also put large fish in little 10g tanks, while most of the LFS that I have been to have more appropriately sized tanks for the larger fish. It's my experience that the folks working at Petco are not very knowledgeable and don't really care about the livestock.
And you're right, people can spend their money wherever they want, but I personally would want to hear about bad practices, like this thread is telling us, so I can make an informed decision as to where to avoid getting sub-par livestock. I would expect to see that about any other vendor as well. That's why we have the vendor experience forums.