So let me get this right, if JPW thinks he's out of bounds, but the whistle hasn't been blown, the linebacker should automatically assume JPW is out of bounds and let up?

Here's the bottom line: JPW let up too close to the sideline and got reamed for it. Next time, he'd better make sure he's clearly out of bounds before he gets off of his horse. I wouldn't ask any less of my linebackers if I were a coach.
Here's a scenario:
Mike Shula: "Hey, Juwan! If it even remotely looks as if a quarterback, receiver, or running back is near the sideline and he slows his speed, you had better not hit him!"
Juwan: "But coach, the whistle hadn't been blown, I was running full speed, and what if he'd cut it back upfield for a couple more yards?"
Mike Shula: "I don't care, Juwan. We don't want to run the risk of a penalty being called. Next time, just hope he runs up the middle of the field so we don't have to speculate whether to hit him or not.":rolleye2: