Once again, you misunderstand. The rules literally change at black belt.
Colored belt - can punch or kick to body. Cannot punch or kick to head.
Black belt - can punch to body, can kick to body or head. Cannot punch to head.
The question in this case is should a colored belt start training for the strategies that will be used in black belt competition, or should they focus on their level. In the scenario you get your black belt in June and there is a tournament in July, do you:
- Put your red belt back on, because you haven't been training to set up or defend against headshots.
- Have been training to set up and defend against headshots since red belt, because you were training ahead.
- Go to the tournament, having only spent a month training to set up and defend against headshots, because you only train at your level and you will fight at your level.
- Not compete at all, and wait until next July, because you haven't been training headshots and aren't ready to compete.
As to what people can or can't prepare for by testing, that's ignoring:
- Mixed classes, where lower level students try higher things
- Progression of the technique as it appears in the curriculum (i.e. our green belts get an introduction to the spinning hook kick, blue belts are expected to make it look like a kick, and red belts are expected to be proficient in it)
- Different learning speeds and time-in-grade requirements
We have some students that take 8 months or more to get through the material required for one test. By the time I got my green belt I knew 95% of what I'd need for my black belt, from watching the other classes. We have some students that take years to understand how to do basic kicks. I landed a 540 roundhouse on my 3rd try as an orange belt, and we have a girl who was doing head level double-back-kicks within seconds of me showing her how to do them.
People learn at different rates, and just because someone is capable of looking ahead, doesn't mean the curriculum is bad.