Once they look healthy they are perfectly fine out in the open.
I line next to a commonage where horses are kept. When I moved here I thought they were wild horses, but soon got to understand the rules of a commonage and that they belonged to various different people and this was their grazing lands.
I had similar concerns as I was a city boy and the horses are out 12 months of the year.
I was put right and told they are a very hardy creature and once they have enough fodder during the non growth months they are perfect.
I'm here 17 years now and enjoy watching them.
So unless they look in poor condition and no-one dropping fodder/hay to them regularly, I wouldn't be too concerned.