Why do boys swim with boxers on?

They say it's because they feel naked without it and it's more comfotable. It is in fact very unhygienic since the shorts they are wearing are the same the wear all day. Here in Sweden it's now forbidden to wear it in swimhalls. The clore in the water isn't enough to keep it clean if ppl do that. The level can't be higher since we would get hurt if it was.