No in fact with questions of religion, psychiatry, diets, etc it usually comes to 'large bodies of populations' or even 'majorities of populations', you're going to find 'some' of everything.
Sure. The question is whether your body can sufficiently adapt to voluntary dietary choices that it would be cruel for a prison to feed you food that you can no longer tolerate. If you can find "some" of it, then I would say yes, a mature society should be inclined to treat this as a legitimate concern.
Whether there's any particular guarantee of this happening is another question entirely and completely irrelevant. It would be relevant if you could determine that there is a guarantee this won't happen, but that's it.
