Jails may make an argument that canteen funds should be treated separately from government funds. We would argue that they aren't. Once they enter the canteen it's the government's money and needs to be treated like any other money that belongs to the government.
The best situation of all, and one frequently utilized, is for jails and prisons to allow volunteer ministers of all faiths to enter prisons and offer their services to the inmates who want them. That way, the religious needs of inmates are met but without government funds being spent.