If you think about all the people who buy a tank and a bag full of fish in one shopping trip, then wonder why they all died... if you can imagine those people all coming in and cashing in on guarantees that the fish stores have because the fish store has no means to protect itself (I.E. requiring a water test) then you can also imagine what we'd all be paying for fish when these fish stores have to roll those losses into their operating overhead.
I even know one guy I work with who wanted to get his kid a tank for Christmas. I recommended that he go to the store I go to. He goes there, tries to buy everything at once, and they wouldn't sell him the fish (a good thing IMO). He goes home, sets up the tank, then sends his wife in later that day to buy the fish. A week later, half his fish are dead and he's back at my LFS trying to cash in on the guarantee. Luckily they tossed him because of his water test. All I can say is that I'm glad that I am not paying for his ignorance every time I go in there through higher prices. Granted it's somewhat my fault for sending him to my place, but he's the one who tried to scam the store.
I agree that having your LFS test your water regularly, particularly if it's a mom and pop operation where you know them and they know you, is a good thing. In the rare instance that I have to claim a guarantee from my LFS, I don't even have to bring a water test, they just know my water is always good.
While I do not disagree that some fish stores may use their guarantee as a way to circumvent making good on their guarantees, you can't condemn the industry for the sins of a few.