They pay to maintain them and provide staff. These companies promote products to which many parents object. Take violent entertainment, for example. They certainly don't want schools to promote entertainment that glamorizes violence and gore. Another example is junk food.
Childhood obesity is a major public health problem.
The public schools. Channel One promotes a parade of junk food and fast food to impressionable children. Coke and Pepsi, which have a major presence in many schools, are laden with sugar, excess calories, caffeine, and other additives that can contribute to obesity, tooth decay, and many other health problems. On top of all this is the question of values. There is growing concern about the moral atmosphere of the classroom.