As someone who grew up in country with such policies it is easy - it creates bad incentives for school cafeterias. They can push quality of food down, because t

As someone who grew up in country with such policies it is easy - it creates bad incentives for school cafeterias. They can push quality of food down, because they have monopoly for government food money. So the food is barely eatable, you could buy much better food over the street (or just eat at home), but then you have to pay the full price. Also it discriminates against people with strong dietary preferences. E.g. if you are a vegetarian, and school cafeteria does not offer vegetarian food, it is no government food money to you. Also, fair question is not 'free food in school cafeterias or not', the fair question compares equally expensive alternatives: 'should this government money go to parents as child benefits, or to school cafeterias for free food'.