For more than 130 years, the default rule in employment in the United States has been the rule of employment-at-will. Persons who are employed-at-will can be fired for a good reason, a bad reason, or no reason at all - even a totally arbitrary or irrational reason - as long as that decision is not unlawful as a result of a specific law, such as the National Labor Relations Act or federal, state or local anti-discrimination statutes and ordinances. In contrast, most unionized employees in the United States can be fired only if the employer has a good reason, or just cause, for that decision.
The alternative to employment-at-will is a simple one, a requirement that an employer have just cause to discharge a non-probationary employee. That is the law in other advanced industrialized countries. Highly compensated corporate executives, athletes, persons who work in broadcast journalism and others with highly specialized skills often enjoy the protection of just cause protections in their individual employment contracts. So do the majority of unionized employees in their collective bargaining agreements. A number of academic commentators have proposed that the federal government or state governments adopt just cause statutes.