Firstly, I think you hit the nail on the head with "So that obviously acknowledges that there is a need to keep players away at times." But, the key words there are at times. All I'm trying to say is that Bungie wanted a consistent open world, but they've been very deliberate in referring to the open world as a cross roads. The idea behind the game is to have that social link by crossing paths on your way to your missions, and perhaps being drawn into a public event on the way, etc. Bungie shut off public access in story areas, for the very reasons you describe, such as people interrupting the story and whatnot. However, Bungie did not make the crossroads optional, because it's the idea behind the games shared aspect. Without it, a large part of why they made the game kind of goes away. The approach it seems they're taking is not to please everyones social taste, but to make the game they want to make, and offer it to those who want it.
I understand what you're saying, but I feel like it would be against the purpose if they gave that option, because they want all players crossing paths in the public space at all times, always interacting. To give an option to turn that off would segment that vision to players who are okay with the public spaces and players who opt out, fragmenting their unified world. It's the same reason there's no "play offline" option.
Yes, at face value giving the option to avoid public spaces doesn't hurt anyone, the same that the ability to turn off the internet connectivity doesn't hurt anybody. But in actuality, it hurts Bungie's pretty focused vision for what this game is supposed to be, which is why it isn't going to happen.
Also, from a purely artistic perspective, there's a concept behind the game that the artist does not want jeopardized, no matter who would enjoy the alternative options to the concept laid out by the artist. But that point leads to a whole other conversation about entertainment and art and all sorts of things.
