Have you ever read Peter Drucker as part of getting your MBA?  He said most MBAs are educated beyond the ability of their common sense to handle their education - he's proven right again.

Besides, the testing is a good way to weed out (pun intended) stupid job candidates.  There's a great big sign posted right there that says you will be tested for drugs and they apply anyways.  Why waste their time.  This also helps to eliminate people with no reading comprehension.