From the official press release:

Players will delight in producing fantastical and sometimes hilarious interactions between the Mario and Legend of Zelda characters and objects, alongside everyone’s favorite word-wielding Scribblenauts adventurer, Maxwell. Discovered Nintendo characters and objects will behave in the fun and fantastical ways they do in their respective game worlds.

Nintendo’s number one asset is its library of iconic characters, so what better way to give third party publishers a boost than by loaning out some of those characters for a Nintendo-exclusive experience?

Loaning characters out to every Tom, Dick, and Harry may not be in Nintendo’s best interest, but Nintendo can still lay out some guidance and insight into their philosophies to third party developers in order to help create that unique Nintendo experience consumers look for when buying a Nintendo product.

I hope to see more of this kind of thing in the future. When Nintendo starts working with third party developers and publishers to create experiences unique to Nintendo platforms, everybody wins.