I think it is because people do not realize what they are doing when they are calling Hatzolah.
Hatzolah is a great organization, they are here for our emergencies or for patients that cannot get to the hospital without an ambulance.
Calling Hatzolah for non emergencies, such as when the doctor tells someone with abdominal (and no other symptoms) pain to go to the ER and have it checked out. This is clearly not an emergency (otherwise the doctor would call Hatzolah from his office), it makes no sense to take an ambulance with a full crew out service for this non emergency. (patient could have driven to the ER instead of driving home and calling Hatzolah).
This makes no sense, especially if the patient demands to go to a long distance hospital.
Again, i am not advising anyone not to call, however keep in mind that hatzolah is an organization run by volunteers and are here for emergencies.