The question you pose is really a little more fundamental than the way it seems on the surface.
Lets look at it. You agree that two parties to a business arrangement can make a contract. They can agree on conditions of employment. For example, what to wear, etc.
But they cannot agree on other conditions. Like drug use and verification of the terms of the agreement. On those items, you seem to say that they are not allowed to come to agreement, that somehow the use of force by a third party, in this case government, will be invoked to make sure that one party does not insist on terms which the other party finds objectionable.
In a society where free association is valued, and no one is forced to make an agreement they find objectionable, neither side is forced to make the deal. Either side may decide not to be involved.
People who are libertarians do not believe in the use of force to get what they desire. The issue is one of free association minus force.
I understand the concept of free association and contract. BUT there is the "force" of hunger. Taken to it's logical conclusion ... (and which is one of the reasons that I am not a Libertarian). Thud industries does not hire women ... the jobs they have could easily be performed by women but they feel that a womans place is in the home. The prevailing political / socialogical atmosphere in this mythical world is that all men (very Taliban like huh) feel that women should not work and NO industry or company will hire them. You see in the Libertarians view that's perfectly ok due to free association. I DON'T BUY IT. You would say ... oh the women could open there own business ... sorry no company does business with a company that employs women ... so now what. A person has to eat/work/have a roof over their head. I know that it's NOT in the constitution but I'm a fraid that there are several modern day circumstances that are NOT covered by the constitution. I'm not saying that the constitution is a living document ... BUT some common sense has got to apply with a 230 year old document that could NOT possibly be expected to cover nor anticipate all the circumstances of todays modern world.
A simple solution to my anti-drug testing position is to apply the 4th amendment to business (as it should be in my book) That is ... unless it is a critical circumstance ... no unreasonable search and seizure by ANY entity.