the radiation dose level is lower for good reason (wont get into details here). If something suspeciou is found, then they will have to do a Diagnostic exam which typically involves ultrasounds, or even biopsy, or MRI. These procedures may require some co-payment, depending on insurance.
I am not saying there is no fraud, but that was the reason for the follow up exams.
As for ultrasound, it is NOT a good screening tool. So no good for breast cancer screening.