Well a runner generally would run through the bag and angle out towards the coaches box but since Craig slid he stood up on the inside of the bag. On the replay there is no where for Middlebrooks or Craig to go at that point. Craig has to go in a straight line towards the plate which is where Middlebrooks legs happen to be. Craig is facing away from the plate watching the ball and as soon as he turns towards the plate and makes his first step it is too late to avoid colliding with Middlebrooks, his momentum was carrying him. Middlebrooks actually appeared to help him out by just flattening out.
That rule isn't going anywhere though, if you impede the runner whether or not it is your fault it is going to get called. Not saying you don't know that but my facebook page was blowing up over this thing being a travesty. To me it wasn't even a controversial call but I'm also a Braves fan that defended the "infamous" infield fly call last year.