I agree with the fact that it is problematic to say that prisons should accommodate any kind of voluntary diets (vegetarian, vegan, halal, kosher, etc.). I do not say this because prisons should "punish" inmates, but because it would require to draw a line between "common" or "reasonable" voluntary diets, and other diets. (Of course, this does not apply to special diets required for medical reasons.) In fact, I have the same problems with voluntary diets being honored in other State-run facilities such as school canteens.
However, I do not see why, as a general principle, prisoners should not be allowed to buy food and have it shipped to them, using their own money.