At our barn (in OH), people normally only blanket their horses if the weather is really cold (winter time) or if it's rainy/wet and a little chilly (just for some horses). Normally all you see at the barn during the summer are light weight fly sheets if anything at all. (oh and fly masks of course!)
Sandie grows a pretty nice winter coat, so she only gets blanketed during winter if it gets really chilly outside. And I only use a medium weight blanket, she just doesn't need anything heavier.
I think it really greatly depends on the individual horse.
I cannot fathom how a horse deals with a heavy blanket on them over the summer months - oh my gosh, talk about heat stroke. How uncomfortable.
Fly masks - no. You cannot generalize that every horse in the U.S is blanketed or not blanketed. Depends on the circumstances.
As I said, I don't rug unless I have to. Horses are much happier being left alone if they don't need human assistance, and most of the time, people think that because they are cold, their horses must be as well - which is garbage.
We as people, have a bad habit of humanizing our animals and putting human emotions on them. Just because we feel something, doesn't mean our animals are.
Fly Masks - again, depends on the owner and the horse.
Nelson is completely bare all summer. No Fly Sheet, No Fly Mask. He is just happy as can be being left alone without all these human gadgets being tossed on him. Now if he was getting bitten up by bugs, then I would put a sturdy fly shee on him. If he was really sensative to insects, I would put a fly mask on him - depends on the horse, and the owner.