Take it from Nissan's side, the more people they give these away to the less people come in for factory servicing, sure not everyone who got a free manual would choose factory servicing but it's still going to stop some people.
Technical manuals are usually different to the manuals supplied with goods brought, you don't get a full service manual with every fridge you buy you get a manual good enough to show you how to use it, specs and maybe some trouble shooting. If you want wiring schematics and exploded diagrams to dismantle it you need to pay extra or hunt one down from somewhere, so why should car manufacturers be any different?
As has been spoken about several times already for $10 you can have your own copy of these manuals brought off fleabay and delivered to your door. So what if Nissan aren't the ones giving it to you, for that price it doesn't make any difference it's still a bargain if you want one.