Amendment IV of the Federal Constitution - 
"The Right to Privacy"
The right of the people to be secure in their persons, houses, 
papers, and effects, against unreasonable searches and seizures, 
shall not be violated, and no warrants shall issue, but upon 
probable cause, supported by oath or affirmation, and particularly 
describing the place to be searched, and the persons or things 
to be seized. 
Piss tests in and of themselves are illegal if there is not
probable cause. So if the employers don't do this illegal 
practice of violating one's person (by analyzing their urine)
than it would not be a problem.