D&D 5E How can the first campaign at release be FR without a setting book?

[Sword of Spirit]

Take a look at this article from December:

http://www.wizards.com/DnD/Article.aspx?x=dnd/4wand/20131218

Especially this example:

"Part of our attempt to focus on the whole multiverse in the new rules affects the way we look at subraces like this. Here’s another excerpt of unedited text that might or might not appear, in some form, in some unspecified future rulebook.

As a hill dwarf, you’re strong and hardy, accustomed to a difficult life in rugged terrain. The gold dwarves of Faerûn in their mighty southern kingdom are hill dwarves, as are the exiled Neidar and the debased Klar of Ansalon. . . .
As a mountain dwarf, you have keen senses, deep intuition, and a mastery of armor made from the metals mined in the mountains. You’re probably on the tall side (for a dwarf), and tend toward lighter coloration. The shield dwarves of northern Faerûn, as well as the ruling Hylar clan and the noble Daewar clan of Ansalon, are mountain dwarves.
"​

So no, Forgotten Realms won't be the default. Default is primarily going to be traditional D&D multiverse (with tweaks). Instead, they will tell us in core how the options presented apply to a variety of different worlds (ie, which dwarf subraces from different settings are hill dwarves versus mountain dwarves), and they will quite likely map the common deities to the domains included (as Mike Mearls implied in a tweet).

I personally really like this method of presentation.