I guess they should ban pieces of wood and bars of soap too.
https://www.npr.org/2016/09/06/492269332/from-homemade-weapons-to-inmate-art-an-inside-look-at-life-behind-bars
https://www.telegraph.co.uk/news/worldnews/northamerica/12011248/Prisoner-made-fake-soap-gun-in-foiled-escape-
plot.html
I was just going to bring this up, but you beat me to it.