Windows have several other issues, aside the cost. First, people can look out windows and be distracted. This is common on fall days when there is a view outside the windows, or on snowy days. It is more entertaining to watch it snow than to listen to the same rubbish for the 50 millionth time. Cutting out the windows allows that to be cut off.
Another aspect is that one can get spiritual energy from the sun. This may explain why they close the blinds or shades even when they do have windows. Not letting in the sun cuts off that source of spiritual energy. Fluorescent lighting is also very common in Kingdumb Hells--and I would guess that the sick fluorescent lights will remain even if LED technology is perfected. Given the choice between LED tubes that last 20 years, put out 3,000 lumens for 12 watts per tube, emit a light at 6500 K color temperature with a spectrum that is very close to sunlight, and doesn't cause headaches--and fluorescent tubes that are fragile and have mercury, last only 3 years, put out 3,000 lumens for 32 watts per tube, and a 2700 K color temperature with a crappy spectrum and causes headaches, they would choose fluorescent. Also, the ballasts emit dirty electricity that creates interference with your thinking (LEDs don't). (This can be proven with a AM radio--static while the lights are on indicates this issue). And, fluorescent lighting can induce headaches that they can imply "We need the New Order" with. Sunlight and quality LED lighting doesn't.
The whole thing is about preventing people from thinking. You can't think well under fluorescent lighting; you can under daylight. With nice big windows, it might be possible to conduct the boasting session with half or all the lights off if the sun is shining. However, that would stimulate people's pineal glands, which in turn would get them thinking that the whole thing is rubbish. Fluorescent lighting will not stimulate your pineal gland (though high color temperature LED lighting might), and there is no temptation to turn off the brain fog machines (lights) to save electricity if there are no windows.
Another factor is the HVAC. Windows, especially ones that can be opened on mild days, creates ventilation. This is the best way to cool a building on mild days (when the temperature is around 10-15 o C). Using air conditioning or duct ventilation is inefficient, generates positive ions (bad for thinking), and prevents fresh air from reaching people. Toxins (suit dry cleaning chemicals) can build up, clouding your thinking even more. (Plus giving you another source of headaches). Along with the positive ion generating fluorescent lights, this creates more brain static. You leave the place with a headache. This is perfect for the people on the platform to induce a trance, and you can be taught rubbish while in a trance. Jar people out of that trance through 6500 K LED or solar lighting, fresh negative-ion laden air, and real daylight, and they will reject the rubbish.
Conspiracy? Maybe. But, one thing that remains fact--I never failed to leave an a$$embly with a headache. As I generally do not get migraines with the headache (they always come without the headache, and have become extremely rare since I left the religion even without headaches), I know the source of these headaches is the combination of the pxxx poor lighting, stagnant air, positive ions, and being around the influence of people that are an angel magnet. While daylight and fresh air cannot cure the angel problem, at least they could limit the headaches.