Thank you for your comments. I am 100% in agreement w/ you that employeers have no buisness inside your body and shouldn't care about what you do at home. pre-employment and random drug testing is an invasion of privacy and a violation of the 4th amendment. Fight uncostitutional searches and seizures wherever they occur.
As for why companies will waste money drug testing all perscpective employees, I believe insurance companies will give discounts to buisnesss that can claim to be a "drug free" workplace b/c insurance companies have cooked up statistics claiming that "drug free" workplaces have fewer accidents and pay lower premiums. Also, the gov't may give breaks to corps that are "drug free" for similar reasons (we love to reward the status quo, even if it's unconstitutional).
More info for anyone interested, including test regimines, false positive lists, and ways to beat drug tests at
http://www.erowid.org/psychoactives/.../testing.shtml
Information = Freedom