That is a quote my advisor has said time and time again. He has a PhD in History, and has been teaching for over thirty years. His explanation is that you cannot use the morality for the past as a tool for modern morality of the present when teaching history.
Is there some truth to this?
Let me know.
What you're talking about is often framed as "presentism." There are several older threads on this which may be of interest to you:
https://www.reddit.com/r/AskHistorians/comments/4xug1s/monday_methods_the_past_is_a_foreign_country/