i wouldn't go so far as to say its a "myth." yes, a more freeflowing header and exhaust is great, but why doesn't every aftermarket exhaust come in 3 inch trim diameter. its because too much flow can change the combustion cycle by creating too much flow. this will lead to reduced power in lower rpm's.
as is the case with most applications, a compromise is best. stock exhausts are designed for the consumer wanting a quiet, yet efficient design that serves its purpose well.
Why would they float at 0 back pressure? That doesn't make any physical sense. Technically 0 back pressure is not possible, there will always be some resistance to the flow or gases out of your engine.