Well, the context is a facility where security must be so tight that you're air-gapping computers in the first place, so a lot of the same reasons nobody would be permitted to bring their personal film-camera into the place either: You don't want to make it easy for people to take pictures of arbitrary things (faces, documents, whiteboards) and you don't want to always be searching everyone's underwear for instant-photos or film-canisters.

In contrast, a worker can sign-out a hardened device, and when they return it on the way out you can be reasonably sure they couldn't have easily made copies. Plus the scanner won't capture arbitrary pictures in the first place, and it can be set to auto-wipe after X minutes of inactivity.

If you give people 10 unexposed sheets and require them to return a total of 10 used/unused on the way out, that's susceptible to them smuggling in an unexposed sheet, and you're back to underwear searches again.