Just because you don't happen to share their values doesn't make this the same thing as being a picky eater. I don't think you'd be so cavalier if the prison were doing something that goes against your values — for example, if the prisoners were forced to eat human meat.
You're in prison because you disregarded the values that society put in place, surely the society has no obligations to respect your values in prison (although it still goes to some length to do that).
"You shoplifted an expensive purse, so now we can force you to eat your late mother" is a positively medieval stance to take. Prisoners are still human beings, and forcing them to do horrible things like that is a violation of both the general idea of human rights and several parts of the US Constitution.
My point still stands regardless of the extreme example you decided to illustrate it: you disregarded society's values and in return the society will disregard some of your values.
The prisoner doesn't get to decide what is cruel and unusual punishment to them.
