I would generally agree with this, but I've seen most "mom and pop" stores where the fish are FINE, and I'm sure their fish are shipped from somewhere too. Could be smaller shops tend to buy from places where the fish are housed better, but still. I'd support a small local establishment before I'd go to a chain store.
It's a matter of scale. When you have 2-3 employees covering a store dedicated to selling fish (typical "mom and pop") a lot more effort can be put into fishing out dead fish or quarantining them until they look healthy. When you have a dozen employees spread across a store that sells everything, that have a number of different jobs to do in numerous departments, things go unnoticed much faster. It also has a little to do with layout. Most Petcos and Petsmarts are laid out in a way where it isn't easy to scan the whole fish section for dead fish. Chain stores lay out their tanks based on looks and style, not necessarily for ease of checking for dead fish. Mom and pop establishments tend to layout their tanks for ease of maintenance and space, which makes it very easy for the employees to simply scan half the store at a glance and find dead fish.
It is entirely possible to run a saltwater tank without a skimmer. It isn't a requirement at all. Many people (me included) run tanks without skimmers. It just means the aquarium owner has to keep up on water quality more. A high-end reef tank (especially an SPS tank) can be rather difficult to run without one though.I've had the same experience. Sometimes the people are very knowledgable and can help me, then other times I go in and the guy tried to convince me to set up a salt water tank and told me I wouldn't need a skimmer.