Originally Posted by GenuineRisk

If you work for a private employer and you are not a member of a union or under a contract, you are considered an "at-will" employee and can be fired for any reason the employer chooses that is not specifically prohibited by law. Refusal to submit to drug testing does not fall under things prohibited by law.

And if you don't like it? Find another job.

As someone else mentioned, the Bill of Rights does not apply to private sector employment.