I'm surprised Bridgestone lets Nissan install inferior versions of tires. My confusion is an example of how this can hurt their reputation.
It's pretty common for a manufacturer to negotiate with a tire maker for lower priced, lower spec OEM tires. Toyota had a deal with Continental for years, and may still. Camrys used to come with Continentals that were rock hard and had terrible wet traction. They may still, for all I know.