Correct, that is because most engines have roughly an 8-1 compression ratio. The only reason to run higher octane gas is if you have a high compression engine. If you run high octane gas in a normal compression engine, not only are you wasting your money on more expensive gas, your actual gas mileage will probably suffer a bit too.
Many turbocharged cars will run higher compression ratios and require premium. Also, many European models require it as in Europe - what qualifies for 'premium' in the US is equivalent to the lowest octane fuel generally available (91 R+M/2 - 95 RON).
My wife's Volkswagen has a 2.0 Turbo I4 used by many Volkswagen / Audi models. It requires premium.
I'm curious as to why mileage will suffer in a car tuned for regular, but running premium? I agree there's not necessarily going to be any benefit, but I can't see why there would be any detriment either. I would think it goes like this:
Two cars - 1 tuned for 87, the other tuned for 91 are filled up with regular. The engine management module in the 87 car detects no knocking so runs as normal and produces its maximum power. The 91 car detects some knocking so retards the ignition a bit to compensate and suffers a loss of maximum power.
Next, both cars are filled with premium. The engine management module in the 87 car detects no knocking so runs as normal. The engine management module in the 91 car also detects no knocking and so runs as normal and develops its maximum power.
So while there's no advantage to using premium in the 87 car, I don't see where the disadvantage comes from (other than wasted money). That said, the holes in my mechanical knowledge have been cruelly exposed on this board before.
