Now, that's interesting! Since Ford built in a disable feature, they must have anticipated that some owners would dislike the false engine noise and want it disabled. So then why would they design in the false engine noise in the first place?
I can envision a discussion between the engineers and the marketeers:
Marketing: We need this truck to sound more manly.
Engineering: What? Why?
Marketing: It'll sell better if it sounds tougher.
Engineering: Well, maybe. We've never had to do that before.
Marketing: We think it's a necessity on this particular model.
Engineering: We've done all we can with the exhaust system.
Marketing: What about the sound system?
Engineering: You want us to modify the sound system to produce engine noise in the cab? What's the point of using sound deadening materials all through the cab design if we're going to artificially make it noisier?
Marketing: Do it.
Engineering: Yes sir.
(And as they're leaving the meeting, the engineers decide among themselves to program a disable feature just in case the Marketing guys are full of shit.)