Prisoners are human beings too, they need to eat and they need to eat what is right to keep them healthy. If a prisoner has allergies, intolerances, diabetes, anaphylaxis, or another medical condition they need it.
Also for those who have certain religious beliefs. Prisoners have already been deprived of their liberty, the days are long gone where it is appropriate to feed them bread or water.
I agree that if a prisoner is allergic to something, has diabetes, or can be physically harmed by a food, then they should not be fed those foods; but I don't think that is "requesting a special diet" because there is no request, it is necessary for survival. As far as giving a prisoner specific foods for religious reasons, I disagree (and I am Kosher). I think that once you have been convicted of a crime, your liberties are stripped away from you and that includes choosing your food. A conviction means that you are guilty of a crime, a crime is something that you committed by taking away the liberties of someone else (robbery, violence, murder, etc) and you are punished by YOUR liberties being taken away. I'm all for freedom of religion, but that is a choice or belief, and once you are in prison, those choices are taken away in the same regard as the freedom to leave, communicate with family, and generally do what you want are. There are no "requests" or any "special treatment" in jail. You are a criminal and should have thought about your "special needs" before you denied them to the person you went against during your crime; they weren't afforded any "special treatment".
They are not entitled to any luxuries. If they have a medical condition, they should be checked by a doctor, who would give advise, and the convict given just the basic diet they need to avoid serious health problems.
They caused trouble in society, and are not entitled to anything anymore.
Being sent to prison should involve being thoroughly punished for the crime that you have done wrong and that should mean being given no more or barely more than the basics. Should we really have a world where people who commit the worst crimes have the opportunity to actually have benefits from a stay in prison that can actually bring comfort to them during their stay. What a kick in the teeth that is to the person or people that have been badly affected by the crimes that some of the prisoners have committed. A stay in prison should involve a lengthy period of time being deprived of any form of luxury. Instead they should spend their time thinking of what they have done wrong and they should be faced with showing remorse for it.