It's the people who are unaware of the DLC being only free for a week that is the issue. Obviously we all know, but not everyone does. A voucher in the game copy would allow them to get it free later down the line. But this screws them out for not being constantly up to date on the news.
Oh. I guess I don't really see the difference between that or the physical vouchers that would probably run out relatively quickly anyways. If this is how it seems, it'll probably benefit even more people than the first-print copy strategy as people will be able to download the DLC and: get the game whenever they want to, borrow the game from a friend, buy the game used, rent the game, etc. all while having access to the DLC without buying P4AU asap.