As a tall person, I actually find this rule kind of comical.
Headrests in cars (in the front) are actually whiplash protectors, not headrests, per se. This stops your head from whipping back too far in an accident and potentially breaking your neck.
Why is it so important that, as a tall person, my neck be as free as possible in the event of an accident? To ensure that I absolutely break my neck should we stop short or hit something?
I thought the reason is so the FA can see all the way down the cabin in an emergency - the same reason why they have those little windows that flip open on the bulkhead?
If it is not a sudden emergency, you would already have taken the brace position so you would be OK.