The answer is: The parents of the children might feel pressure to give permission to the therapist to use their children's data so that she will continue to provide services to their children
When humans receive a certain positive value from other persons, we tend to have the tendencies to reciprocate and conform to the request made by those other persons. We commonly refer this as the norm of reciprocity.
This is what happen to the parents on the free university clinic. The clinic has offered positive values to them in form of free treatment/medical checkup. So, when the doctor propose to use the children's data, the parents feel some sort of pressure to give permission tot he therapists.