Read again - I didn't say "open air".
I specifically mentioned that they are not completely enclosed on the sides and back.
It's not a temperature or humidity issue - it's about security for some species.
That extra material costs money. Just as you aren't getting the tubs or the heating for free, neither are you getting the extra material used to completely close off the back and the sides.
AP specifically states that the reason why the economy rack is the economy racks is because they are cutting down on the amount of plastics used by not fully enclosing the tubs.