No, that agreement is probably private, but Arrowhead's head of support has been all over Shitter discussing how they need PSN to handle the growing pile of support tickets. It's the same reason any studio signs with a larger publisher: the dev gets the benefit of the publisher's support apparatus, servers, advertising, etc., and the devs get to work on the game, rather than building their own support organization and infrastructure.

Borderlands is a good example of this: Gearbox makes the game, but if you click a support link, it takes you to 2K Games' help desk, since they publish the game