The root thing to realize is the engineers are not secluded braniacs who live in ivory towers.
They have constant feedback from sales and marketing and designing a product that meets what Buyers expect, even without having to read the owner's manual.
For the specific issue with octane ratings, often they have to go back to the drawing board and compromise for Market reasons to satisfy what joe-normal buyer is going to do.
They have to tune their engines just so they can remove "premium required" to "premium recommended" to having "regular" statement. This is not for performance, but because the Sales guys are going to say there's no way we can sell this car in our market if it needs Premium, even if we price it slightly lower than the competition.
This isn't done happily, it's because they know that buyers and Sales numbers are what paying their paycheck.
If they build a car that doesn't work in the open market, they are out of a job.
So the round-about point is this:
Be assured, they've already idiot-proofed and designed the car for typical american use. They aren't going to build a funny 1-off car that only runs on European fuel or something like that, and sell it in the US, and have a "gotcha, your engine is blown up" for all the people who didn't read the owner's manual and read an explicit instruction.
Let alone in this case, the OP is deriving implicit instructions.