There's another benefit to Apple as well: engraved products aren't returnable.
As for someone changing their mind, do you really think that's Apple's motivation behind offering engraving?. I'd guess less than 1% of iPods are sold engraved, and of these the unwanted rate would be in single digits, probably again close to 1%, so (with those assumptions) we're looking at about 1 in 10,000 units.
Instead, I'd put forward that Apple offer engraving for some other reason, and the anti-return policy is simply a byproduct of that, not the driving force; an engraved iPod can't be restocked and would be cost prohibitive to refurbish
The mandatory six-month warranty for defects, however, is independent from the money-back guarantee.