I don't think it's a rule, just a recommendation. But then again, I read the evidence and make my own conclusions. I tend to see many things in shades of gray instead of black and white. So if people ask me what food to feed young birds, I'm going to say starter/grower has worked for me. Why feed extra calcium to a bird that is not excreting it in eggshells? Plus, calcium is cheap and I'd rather buy a #50 bag and serve it on the side instead of paying a "feed price" for it.
Funny, I do that very same thing but when I do it, it only ticks off a lot of people.