Why is Merida considered to be a Disney Princess?

All of Pixar's feature length movies have been credited as Disney/Pixar collaborations. I'm not entirely sure how involved the employees of Walt Disney Pictures are in Pixar's movies, but I had a feeling it was the work of more than one studio, and Disney have been doing a bit more than just distributing. It might even just be that some of the individual employees go between the two studios, I've never been too sure. But I think Disney does actually get involved in the production of Pixar's feature length movies.

I also expect she gets away with it because Pixar had already been bought out by Disney by the time the movie was created. I think Brave was pitched around 2008? And Disney bought Pixar in 2006. I'm half expecting a Marvel or LucasFilm character to be added to their princess line-up someday. XD