You can stop them from wearing it by clicking on the backpack in the inventory.
That I have noticed, I seem to be doing it at 3AM or a time like that (early to bed,
very early to rise).
I think I have noticed that the social groups affect how they wear them, Agnes was a level 9 Jock by the time I realised she was wearing it one strap, but I'm sure when she arrived it was both straps.
That makes sense, reminds me of a movie I watched recently (won't mention it, it's not the most family friendly movie in the world).
I would've thought they would function as some kind of storage space, though, even if only for the realism.
Thanks!