There seems to be a lot of speculation here. How about a few concrete sources?
From the U.S. Department of Labor:
The Fair Labor Standards Act (FLSA) does not require payment for time not worked, such as vacations, sick leave or federal or other holidays. These benefits are matters of agreement between an employer and an employee (or the employee's representative).
From the Missouri Department of Labor & Industrial Relations:
Employers are not required to provide vacation pay, holiday pay, or severance pay — these are benefits given at an employer’s discretion. The exception would be instances where an employer has entered into a contract where certain benefits are established by agreement.
Employers are not required to offer paid sick leave or any other type of paid fringe benefit; this is left up to the employer’s discretion, or to any contract the employer may have with its employees. Likewise, an employer may provide sick and vacation benefits to one group of employees, but not to others, as long as the employer does not discriminate based upon age, race, sex, religion, national origin, etc.
From the New York Department of Labor:
Q: Must an employer pay workers for holidays, sick time and/or vacations?
A: Under the New York State Labor Law, payment for time not actually worked is not required unless the employer has established a policy to grant such pay. Holidays, sick time and/or vacations fall under 'time not worked.' When an employer does decide to create a benefit policy, that employer is free to impose any conditions they choose.
From the California Department of Industrial Relations:
There is no legal requirement in California that an employer provide its employees with either paid or unpaid vacation time. However, if an employer does have an established policy, practice, or agreement to provide paid vacation, then certain restrictions are placed on the employer as to how it fulfills its obligation to provide vacation pay. Under California law, earned vacation time is considered wages, and vacation time is earned, or vests, as labor is performed. For example, if an employee is entitled to two weeks (10 work days) of vacation per year, after six months of work he or she will have earned five days of vacation. Vacation pay accrues (adds up) as it is earned, and cannot be forfeited, even upon termination of employment, regardless of the reason for the termination. (Suastez v. Plastic Dress Up (1982) 31 C3d 774) An employer can place a reasonable cap on vacation benefits that prevents an employee from earning vacation over a certain amount of hours. (Boothby v. Atlas Mechanical (1992) 6 Cal.App.4th 1595) And, unless otherwise stipulated by a collective bargaining agreement, upon termination of employment all earned and unused vacation must be paid to the employee at his or her final rate of pay. Labor Code Section 227.3 The California Legislature, in order to ensure that vacation plans were fairly and equitably handled, provided that the Labor Commissioner was to "apply the principles of equity and fairness" in resolving vacation claims.
As you see, the standards vary. The federal government doesn't require employers to pay vacation at all, but states may make more specific laws about it. In my personal experience, most employers do pay out unused vacation (except perhaps in the case of a firing), but the fact that they do so doesn't necessarily indicate that they're legally required to.