There is absolutely no logical reason to do this. And what is even more baffling is that people are being just as illogical in actually defending such a terrible design choice if true.
This is Destiny we're talking about here. A game that, although can be played as solo as you want, is still a social experience and a multiplayer game through and through. To further take away the choice for people to want to play a social game however they want, be it alone or with one or two friends...there is simply no reason for it.
And for people arguing that this could be for "narrative" reasons, bull fucking shit. This is Bungie we're talking about here. This is a shooter we're talking about here. This "narrative" isn't going to be any more compelling than Halo was as a shooter and every Halo had coop from the start. And Destiny is more focused on multiplayer than Halo ever was. And for fuck's sake, there should be no technical reason behind it either because Bungie has already been able to pull this off in the past. Don't try to pull that bullshit argument.
But hey, as I said, this is Bungie we're talking about here. I know of no other developer who has made so many design decisions with no real reasoning behind them besides ruining the fucking fun of the game.