The major argument against such testing is that it invades the privacy of the worker. Because it does this, one can argue, it should not be allowed unless there is good cause to suspect an employee of drug use.
In our free society, with its protections for our personal rights, we generally do not allow people to have their possessions or their homes or their persons searched unless there is a very good reason to do so. We can argue that this should apply to employers as well. Employers should not be able to simply force workers to submit to intrusive tests unless there is a good reason. If we allowed such tests, you can argue, we would be opening the door for employers to try to take more and more control over the lives of their workers.