It's a thing with some teachers in some places. The quality of education in the US is hugely variable, because standards and curriculum are largely left up to local school systems with widely different funding and priorities.

That was a short answer that turned out to be mostly wrong. The longer answer is "if you don't understand how this works, you won't have the intuition to notice when you get absurd results from the calculator". If you don't have that intuition, then when you inevitably make a small usage mistake on a calculator (or in matlab or wolfram alpha or whatever), you'll end up not realizing that you got a clearly wrong answer.