I'm rooting for the Sox and I can't say that was a bad call. It was a terrible throw by Salty that set that interference up. I'm not so sure there wasn't intent by Middlebrooks to trip him. It almost looked like a defensive player in football making a tackle with seconds left in the game who lays on the guy to help run the clock down.
There are so many rules in sports that don't make sense to me. A runner can interfere with the defense and it's ok but the defense can't interfere with the runner. I'd much rather see no interference allowed at all. It's baseball, not football.