Any parent has the *right* to complain to a school when they object to anything the school is doing. In fact any paying customer, has the right to complain about a service that they are paying for.
The school also has the *right* to ignore them if it CHOOSES to (so long as it is not breaking any legally binding agreement.)
In this instance the school obviously CHOSE out of financial self-interest, to retain the paying custom of these parents...
I guess like most private services, the school wishes to provide a service that will satisfy it's customers, and not have them taking their custom and coin, elsewhere.