Think it's most likely due to the fact that bungie had been a MS exclusive company for so long, they needed to show some love to sony fans who may have never played a bungie game before, what better way than releasing a few bits of unimportant exclusives their way? Also imagine how much more butthurt sony fans would be than xbox fans have been if it was the other way around, the old xbox company still favouring xbox would not have gone down well