Unfortunately, how close a runner is to the bag has nothing to do with an interference call since you can't assume anything. It's strictly interference on a fielder attempting to make a play. I've had to call a couple of those and as anyone with common sense, you know they have no shot of getting the runner but you also can't ignore the fact contact happened. Had Kepler still been in the box, then he'd have been fine as long as he wasn't actively trying to mess with the catcher's attempt. Tough break.