Batter does NOT have to be completely out of the box. The batter can be called for "any movement" while standing in the box. For it to be interference there has to be a play that can be made. If there is no runner on 3B, no runner going to 3B, etc, then interference cannot be called.
Which has nothing to do with the obstruction rule. It is not obstruction until the runner is impeded by a defender without the ball. Where the defended stands has no bearing on this play.
I love the high strikes. Would rather call them strikes than balls. That's why I call the book, period. If coaches and/or umpires don't like it, get the rule book changed. And yes, I have called these strikes at 18U gold level tournaments. No issues.