The military provide us with a sleeping bag, so they can claim they do provide bedding. This is used on uncovered mattresses, some of which after 2 years + of multiple users are pretty manky as you might imagine. There are pillows, but most of these are pretty grim as well.
So some people are providing their own sheets to cover the mattress, and at this time of year out there where it is regularly 100+, I am not surprised that people also take their own top sheet as opposed to using the sleeping bag! Not sure I'd bother with a duvet at the moment, but people have taken them at other times of the year.