Opinions- too authoritarian?

Today, I was with my son and I explained to him why he was not allowed to do something. To my surprise, my MIL scolds me as to why I offered him in explanation. She said children didn't need an explanation, just to know to obey you. She said once they no longer like your explanation, they don't obey you. I had never heard this before. Any opinions on it? She claimed it established authority over friendship.