A runner is out if:
He fails to avoid a fielder who is attempting to field a batted ball, or intentionally
interferes with a thrown ball, provided that if two or more fielders attempt to field a
batted ball, and the runner comes in contact with one or more of them, the umpire
shall determine which fielder is entitled to the benefit of this rule, and shall not
declare the runner out for coming in contact with a fielder other than the one the
umpire determines to be entitled to field such a ball;

What I gather from that is because there wasn't a play being made on Craig at the time he and Middlebrooks made contact and he wasn't attempting to field or receive a throw, it was deemed obstruction and not interference.