That's not true a bunch of states have provisions against that
Edit :Thirty-six U.S. states (and the District of Columbia) also recognize an implied contract as an exception to at-will employment.[28] Under the implied contract exception, an employer may not fire an employee "when an implied contract is formed between an employer and employee, even though no express, written instrument regarding the employment relationship exists."[28] Proving the terms of an implied contract is often difficult, and the burden of proof is on the fired employee. Implied employment contracts are most often found when an employer's personnel policies or handbooks indicate that an employee will not be fired except for good cause or specify a process for firing. If the employer fires the employee in violation of an implied employment contract, the employer may be found liable for breach of contract.
Good luck proving that the ball boys were working under an implied contract instead of at-will employment.