> Religious diets are not voluntary, but choice of religion is voluntary.
Agreed
> Vegetarian/vegan diets adhering to an ethical philosophy are not voluntary, but choosing to accept an animal rights philosophy is voluntary.
Not agreed. Your right to accept certain philosophies while in prion is forfeited upon conviction. This means, if you are not eating meat because you simply feel it's wrong -- well, the prison has no obligation to oblige you. If you are not eating meat because of your religion, that is different. Sure - one may "convert" to a new religion to obtain a certain preferred diet, but that is still applying the rule evenly and fairly.
> Now, if the vegan diet was for personal taste or even for health reasons (as long as there was no significant health impact from going off of it), then it's a different story.
This would be the foundation for an awful law which would be enforced with bias and randomness. How do you actually determine if a diet is being followed for pure taste reasoning, or if it's something based on personal ethics? You cannot, and therefore this cannot be a rule. Rules and laws must be based on a foundation that can be applied evenly and fairly to all, not left up to judgement by a sometimes-in-a-nice-mood-sometimes-in-a-bad-mood warden.