I have mixed opinions about the (sort of) rule. It was an IAABO-ism that was independent of the NFHS originally, and I'm pretty sure I hated the rule just because of that. But the IAABO guy on the NFHS committee managed to convince a broader national group to adopt the idea, and so they did. Except the rules editor dorked up by forgetting to put it in the book, and the book had already gone to press by the time the oversight was realized.
I digress. Having observed the intent of the rule this year and having occasionally enforced it, I would first say that I agree it's hard to officiate both the cross-lane spaces and rebounding...while also officiating this area specifically. I found on the few occasions I had to call something it was because something clearly obvious (contact, or the shooter racing in early) showed up in the right peripheral of my right eye. Many times I thought the top defender might have crossed the FT line early, but not being 100% sure I would pass on it.
That all said, I'm not convinced the rule that adds a substitute FT is dumb for HS basketball. When a defender does that, especially if he/she has done it earlier in the game, it can be a distraction to the FT shooter. A free throw should be free. Distraction = disconcertion, so this rule merely provides some objective criteria for a particular type of disconcertion, something the NFHS wants eliminated from the HS game. We know it's a bad coaching strategy to "clear out" FT shooters. But unfortunately at the HS level there are plenty of bad coaches. C'est la vie.
Hard to accurately observe and enforce? Yes. But a dumb rule? Not so sure about that.
But I still dislike IAABO.
