I totally agree with this. I usually turn my monitor brightness down to the lowest setting (my work monitor has been set this way for years). I've done the A/B testing on myself and find that I will develop something like a mild headache (probably eye strain) after a day of working at the default brightness. On my old LCD monitor the brightness would reset if power got disconnected so I did the experiment several times accidentally. My current monitor (Dell U3014) remembers the setting across power resets. I have my brightness control set to zero. It looks dim at first but after a few minutes it looks completely normal and you forget about it.
>(the same reason they sell/create/display glossy monitors, because they demo well).
They create very few of them. The few they do create are because some of us want monitors that are not intentionally blurry. I am capable of controlling the lighting in my environment. I do not want my monitor to deliberately make everything fuzzy just to avoid a glare problem I don't have.
Manufacturers likely set this so that the colours 'pop' when being demoed (the same reason they sell/create/display glossy monitors, because they demo well).
Taking the brightness down almost never impacts usability, but I've found can significantly affect eye strain (that 'OMG, my eyeballs are hurting' feeling).