The rules around working with prisoners or other captive audiences are strict because it’s so easy to apply undue pressure to them.
The goal isn’t just to get someone to say “okay”; the point is to make sure they’re doing so freely and with a complete understanding of the possible outcomes.
> "Do animals consent and understand?" -> "No, because they're animals."
Where is the circle?
> "Do prisoners consent to being imprisoned?" -> "No, because they're prisoners."
Is that circular reasoning too? Or is that simply the flat truth, whether you like it or not?
Where you go wrong is in assuming that seeking the consent of animals is a universal value that everybody else shares with you. But that is not true. The answer to "do animals consent to this" is "No, and so what?" The question you pose is only compelling to people who already share your values. To somebody who doesn't share your values, your argument is easily addressed and dismissed. The animals being eaten don't consent, and we don't care because they're animals.
The goal isn’t just to get someone to say “okay”; the point is to make sure they’re doing so freely and with a complete understanding of the possible outcomes.