The chicken will move in or out of heat at all sorts of times, like into or out of shade or sunlight, etc. That's not harmful to their health at all. A bird that wouldn't move to regulate her heat would be in big big trouble.
Why would a hen not have a heat lamp at night, should one chose to use a heat lamp? I don't understand that.
I agree that it's stressful to consider what might happen in a power outage situation, but I think my water pipes in my house would suffer before my "non-acclimated" birds.
I get an impression you think "heated" coops must be quite warm, which is not necessarily the case. A coop can be heated and be just above or below freezing, or colder. I haven't measured mine, but water freezes quickly even under the lamp, and I don't heat at or above about -10C at all. My coop IS heated.....I wouldn't want to sit and read in it in shorts, though.
why heat? Well, for me, for instance, I have small birds and not enough of them to generate enough body heat to heat the space, and I live where it gets very very cold for long periods of time. If any of those things were different, I probably wouldn't heat at all, because there would be no need. I don't want to confine them to only the space they could heat, don't want to get more, and am not planning to move, so I provide them with heat so that they can stay alive and comfortable and not require more calories of food intake than is possible for them to consume. I have absolutely NO guilt about going into my house and leaving properly housed livestock outside.
https://www.backyardchickens.com/a/winter-coop-temperatures
That article does a great job of discussing the hows and whys and whens.
The equation has to be that if your birds can eat enough to produce enough heat and keep themselves at a healthy weight, they don't require additional supplemental heat. If the housing can be changed to meet that requirement, supplemental heat won't be necessary...BUT if they will not be able to metabolically stay warm enough to be comfortable/healthy/alive, then not heating is poor management of their needs.
No, you can't always control things, but if that is a reason to never do something, many MANY things are impossible. I still think providing a blanket statement that "heating is bad" ignores that a LOT of those factors I listed are, in fact, VERY controllable, and shouldn't disuade someone from providing heat
if and only if it is necessary for the health of their livestock. (You heat your own home, which carries the same sorts of risk of fire as a properly heated outbuilding...because the risk is deemed controllable and the preference to be able to live with fewer clothes considered higher priority....you do not NEED to be "room temperature" for your health....)
In general, I agree with what I think you're saying, in that people are quick to heat. That includes their own homes (people acclimate to cooler temperatures as well) and their desire to be able to wear shorts in the dead of winter in -30C weather. In general, the equation is met via feed and birds/square foot. But it's a shame to frighten people into believing that "heating is bad" when explaining the factors in deciding how to approach winter weather would make them more able to respond appropriately to all sorts of situations....(if it's not very cold, but it rains a lot, your birds will probably need more food and different run setup than if it's a bit colder, but dry, if you see what I mean re adaptability through education)