People follow rules even when they don't understand what purpose these rules serve. The heat guidelines for baby chicks are one of the most mystifying to people for some reason.
Here's why you provide heat for chicks. Lacking feathers, and their down being so sparse, baby chicks will lose body heat if the ambient temperature (whatever the room temp is) falls below their body temperature of around 100 F. So we supply a heat source for chicks so they can warm themselves back up as they lose heat from their bodies.
As a chick begins growing in feathers, which they do fairly quickly over the first four weeks, they need less heat under which to warm themselves, and you want to decrease the heat as their feathers grow in so they don't get too uncomfortable and even sick.
Now, if the ambient temp is very warm, say 85-95F, chicks aren't going to lose much, if any body heat, so they really don't need much in the way of additional heat because they don't need to warm themselves if they aren't losing body heat.
It's the same as when we humans go outside on a winter day. We're close to the same body temperature as a baby chick, and if we don't have proper insulating gear, we quickly lose body heat and need a heat source to warm up again. But if it's a warm summer day, close to our body temp, we are fine without any insulating coat and hat and don't need any heat to warm us up.
