I think the missing link is that the prison warden doesn’t even think about the animals - they just think, this is food, eat the food. They aren’t intentionally causing animal suffering, its just a build in part of the system that society as a whole has accepted as normal. So someone not eating the food is just being annoying, in their eyes, regardless of their reasons.
Is this ethical? Fundamentally no, because it accepts animal suffering as a premise. But it makes sense why the prison isn’t accommodating them.