It's the default alright, but that has nothing to do with it being in the base (yes, it's an exception to the rule). You can look it up on the freebsd-ports@, there were somewhat heated discussions concerning this and some other issues. Unless I've missed something, this decision hasn't changed.

pkg can also bootstrap itself. I don't have a fresh FreeBSD install w/o pkg already installed but upon first use it basically goes like this:

  $ pkg foo
  pkg is not installed. Install it? (y/n) y
  ...
  pkg is now installed

Right, that's what I said[1]---there's a shim pkg in base to install pkg proper from ports. The real pkg is in ports so it can receive continuous upgrades, which wouldn't be possible in a RELEASE (or STABLE for the most part) if it were in base.

[1] https://news.ycombinator.com/item?id=7114744