I don't understand this thread at all. If you're trying to recruit a prisoner, you want them to have the best food and be as comfortable as possible so they recruit faster. If you don't want to recruit them, and are just harvesting their organs or selling them or whatever, then uncheck "gets food" and build them a nutrient paste dispenser, as has been suggested over and over in this thread.
Otherwise, what's the problem??
People see no problem with limiting the quality of medical care individual prisoners receive (why waste the best medical care on prisoners when colonists take priority) but for some reason cannot see why the same principle might apply when it comes to food quality. The case and reasons have already been stated above.
Yes ticking 'no food' and having a nutrient dispenser works for some prisoner situations but not all. I may want to charm one prisoner in my cell with skills i want for my colony, so I will treat him well, but want to sell another prisoner in the same cell who has poor skills and I do not want to waste my best food on. Yes I suppose I could build separate prison cells, having one cell with a nutrient dispenser, ticking prisoners who I do not care about as having no food, hoping pawns will fill up the dispenser, and building a better cell without this, but a simple tick box (just as with medical care) is just better. Adding this would help with some players whose playstyle is clearly different to your own.
As an aside, I really don't understand why certain people on these forums constantly oppose suggestions just because they wouldn't use the suggestion themselves. I totally get that stance it if a feature being suggested would impact or drastically change everyone's game style. But if being able to limit meal quality was added then the default setting undoubtedly would be set to highest by default, so it wouldn't even affect those who say they see no problem. [Edit - Alternatively you could add a tick box that was 'prioritize best quality food' (default setting), 'prioritize lowest quality food', or 'gets no food', then everyone is happy.]