Reindeer games, like monopoly

When I was in elementary school, we'd have xmas carol singalongs. Whenever we sang Rudolph the Red-Nosed Reindeer, we'd always sing the callbacks that weren't on the lyrics sheet.

Rudolph the Red-Nosed Reindeer (Reindeer!)
Had a very shiny nose (shiny nose!)
And if you ever saw it (saw it!)
You would even say it glows (like a lightbulb!)

And the teachers would always scold us for singing the callbacks and try to get us to sing it without them.

In retrospect, looking at it as an adult, I find myself wondering: Why on earth did they care if we were singing the callbacks? They weren't by any stretch of the imagination naughty, they just weren't on the lyrics sheet. WTF?