Here is another article from while ago that says the same thing.
http://news.yahoo.com/s/ap/20100607/ap_on_he_me/us_med_overtreated
My personal feelings are most of this is due to doctor's playing defensive medicine. They are afraid of getting sued so they rather do the million dollar workup, than to think critically and only do tests that are necessary. Plus many patients LOVE tests and feel they are getting better care if they get to do a bunch of unnecessary and unrelated tests.
Sometimes when a patient wants to hold off on, or opt out of a test, they are met with hostility from the provider, because again the provider is worried about covering their ass.