"Because I said so" is never alright. You cannot teach children to submit to authority for the sake of authority, you need to teach them to accept critical reasoning by always making that your justification for forcing them to do X. They will never understand if you don't start the cycle of questioning by telling them the reasoning behind your actions.
Obviously. So what do you do once you've explained the reasoning, and are still dealing with a kid whose reasoning is that cookies taste better than broccoli, so cookies for dinner? You say the broccoli is healthier, explain the vitamins and minerals, explain the biochemistry if you have to, the physics if you have to, the mathematics if you have to. Doesn't matter, it doesn't change what they taste like, and that is what the kid cares about. And this is an exceptionally rational child in this example! You certainly can't let the kid eat cookies for every meal; sure, they'll recognize that they feel awful, but that still doesn't change that cookies taste better, and maybe something
else is making them feel bad. There's no proof that it's the cookies; they don't feel any worse after eating them. Why would it be the delicious thing that hurts?
I completely agree that "I said so" is insufficient justification, but I think you're naive if you think that it's never
necessary.
Though, I'd argue that there are tangible benefits to being able to recognize that you should follow authoritative decisions in positions where you aren't equipped to competently argue against the authority. This is why, for instance, I would consider it unreasonable for me to pick a fight with Stephen Hawking about the physics of black holes, or with the IRS about what my tax bracket is. If I invested enough time into researching modern physics, or the tax code and economics associated therewith, then sure.