Who determines when their employment ends?

The employer has the right to fire whomever it wishes. The employee has the right to quit whenever he wishes. Why would one expect an employer to pay compensation or benefits to an employee who doesn’t show up to work? Unions have a right to organize. And employers have the right to fire every last one of them and go non-union. Do you not agree?

Employment ends when the employee quits or the employer fires the employee. However, you are wrong: they do not have the right to fire striking employees. That violates the National Labor Relations Act. In fact, dropping the workers' insurance policy might also violate the NLRA.