Use top tier gas, add a dose of Marvel Mystery Oil to the gas tank every couple of thousand miles and add a bottle of Techron to the tank about 500 miles before each oil change. The amount of money spent is about like a couple of fast food meals so pretty negligible. What is a waste of money is letting a dealer do an expensive "fuel injector flush & service" when doing the steps above will keep the injectors clean.
I haven't had my CX 5 long enough to see if there are any issues with the float in the fuel tank that measures the level giving flaky readings, but in my past 370Z it was a known problem. The addition of a few ounces of Marvel Mystery Oil to the tank apparently had some type of lubricant or solvent qualities that would solve the erratic readings for a couple thousand miles until it came up again. So it isn't just snake oil.
Dammit, Chic-Fil-A is closed on Sundays. Guess I'll keep the money for something way more useful.
Allow me.. "I know what the fuel cleaner doesn't clean on a DI engine. Carbon build-up was so bad on my previous car that I would have needed to get the intake valves walnut-blasted to clean them. I'm asking why Mazda forbids using fuel cleaner in the manual for my current car. Thanks."
To answer OP's question, I think its just CYA on Mazda's part.