I would suspect that once you engrave your iDevice that it would be harder to resell, thus bumping up the market for new devices. Or, it could be that people are more likely to buy them as a gift if they can somehow personalize it.
As a commenter there points out, though, if it's harder to resell your old machine you may be less likely to upgrade to a new one. So there is pressure in the other direction.
The real reason? Apple make more money if you buy from them direct.