Well, obviously they are fine - I'm not saying we ought to rip off people's heads for using a tent.
People might steal things, but at the same time I just either leave everything of value at home, in someone's cartrunk or in my backpack that I always keep with me. The only things of value you ought to bring to a re-enactment are things like wallets, keys, documents and the like, which are small and easily kept in a bag.
You don't actually get a cold from bad weather, that's a common misconception. Obviously you can get hypothermia or something, but you really don't get hypothermia from sleeping outside in cold weather. The human body is quite capable of handling cold weather. Obviously we're not used to it, and yes you might get shitty evening with little sleep, but you get used to it. I can understand people of 30+ getting tents and field beds, because they have a much harder time adapting. But we're young and fit - we're tired and sore for a day or two and then we're fine.
My point is not that everyone should start burning their tent, because I don't care what other people do. My point is that it's actually not thát shitty to sleep without a tent, when done properly, and it improves the re-enactment experience greatly.
Of course, nobody should be sleeping outside, in a tent or no tent, during winter months. Because there was, or there ought to have been, no campaigns in the winter.