Doctors make no money from ordering an extra test. They may save themselves boat loads of time that would be spent in deposition, trial, negotiations should a missed diagnosis be litigated.
There is also a study that shows a slight mortality benefit to defensive medicine. But from a public health perspective how do you charge extra on Insurance for everybody to save a small number of people?
Most large practices perform the blood work, the x-rays, have partnerships with the labs that do the CAT scans. Don't tell me they make no money off the tests.