I agree with others. They are holding an appointment slot, utilizing staff and the room. They are also using supplies, even if the child doesn not agree to the full cleaning. The little bib they usually put on them, gloves for the staff. If they get so far as to run equipment and put the funky toothpast on the zippy brush. If they still offer the toothbrush/floss goody bag at the end, etc. Its still costing them money in all of these ways.
I do agree about the billing to insurance, however. If they, or you, want to try to actualy get the teeth clean, really clean, in a few months, I would ask them about the insurance billing (codes and what not) as most insurance makes you wait until after the EXACT date for your once or twice a year (whatever insurance covers).