If an employer in a free country determines he does not want employees who smoke dope on their own time, they are entitled
Keep in mind that it's not necessarily the employer's preference--the employer may wish to do business the feds, who will insist that the employer have a testing program in place in order to be considered for the contract.
But let's assume for the sake of argument that the virtuous employer hath implemented the policy all by hisself. Why stop with dope? Why not test for...religious belief? Why not require proof of regular attendance at the Krischun Church of Your Choice? How about enforced vegetarianism? How about sound moral practices in video rentals (employer requires you to sign a form empowering him to query your rental records at the local video emporium...oh dear, oh dear, you rented the Director's Cut of Cycle Sluts over the Christmas holiday). But it's a free country, and if you don't think the employer should be allowed to regulate your conduct off the clock, you can always quit. Finding another gig in this economy should be a piece of cake, and if it's not, well, prolonged joblessness is no biggie, eh box?
Pee tests...Stalin himself would grin with derision.
cordially,
[edit: minor typo]