Punishing prisoners by feeding them unappetizing food is also a practice in some American (and Canadian) prisons, although some authorities have been reducing or banning this practice [1]. Since it's theoretically wholesome and contains enough calories, the prisons can claim that they're not doing anything abusive. In practice, it promotes outcomes similar to severely restricting the amount of food served.

[1] https://www.nytimes.com/2015/12/18/nyregion/new-york-prisons...