Question

why are you allowed to use the coarse adjustment when you focus the low power objective lens but not when you focus the high power objective lens?

Answer 1

Because you will focus past ("overshoot") your specimen. (Like using a race car as a shopping cart: it's too fast, and you'll fly past the cereal and never even see it before you realize you need to stop.) Also: you risk crushing the slide and objective against each other (on older or cheaper scopes), and that would be a costly (and embarrassing) mistake.