Beyond dissing the stores, you can see they're trying to create a system to sell fish. One inch per gallon is okay for a fish built like a zebra danio. But once you start looking at fish that are 2 inches long, 1 inch tall and 3/4 of an inch wide, the hopelessness of the formula becomes obvious. No stocking system works as fish have different weight, different movement patterns and different needs.
1 inch per gallon stays around because it tells us what we want to hear. We always want more fish. In general, when we're new to this, we want many more than our tanks can properly hold.
If you step back and consider stores want to make money, and too many fish is profitable, you can see why they'd train their employees to use generous formulas! Overload a tank and you'll need extra equipment, medications, etc, and for a chain that doesn't survive on loyal customers, that's a dream.
I take whatever formula I get online or from common sense, and immediately cut it in half. I don't get invited to many fish parties with that approach, but if a stocking system says you can have 20 fish, get 10.