You are saying that all the reasons are not reasonable, and your one reason encompasses all perfectly.
Ask yourself this question: why do they behave that way? Even a health nut would eat meat if it were the only healthy food available--they would know that the carbohydrate-based gruel and the apples they can get in prison are not an adequate fat and protein source, and will make them ill, and so they will need meat because nobody is going to give them a bowl of nuts and dried soy beans. What, then, would drive a person to such obsession, if not fear of health issues?
Similarly, animal rights and the issue of supporting the meat industry go out the window in situations where your change in consumption effect no change in society. For example, a wedding planner may cater the wedding such that the 40 pounds of meat and 25 pounds of vegetables are vegan-friendly: the vegetables may involve beans, salads, and some better-spiced vegetable dishes, but they'll still be 25 pounds of vegetables, and you'll still see 40 pounds of meat at the reception. Obviously, the prison system isn't going to reduce its ordering of meat to deal with the vegan population. In both situations, what ties you to adhere to a hollow principle? The "animal rights" and "support of animal torture" considerations are founded on sand, and the sand has shifted away, and yet you still attempt to stand firm on air.
In these situations, when a person experiences any distress at the consideration of consuming the proffered meat, that distress stems not from a violation of the world around them, but from a violation of themselves. There is no other explanation: it damages their image of who they are, which weakens their sense of social position. You offer only peacock feathers and dismiss the idea that the bird may have flesh beneath them; I offer that a peacock is a bird of flesh and fat, and the feathers are only the dressing within which it enshrouds itself.