A staffing agency had previously offered health benefits to all "corporate exempt level employees" and not to non-exempt employees. After a payroll audit, it was discovered that all corporate employees were not technically "exempt" employees. As a result, they have to also change their health benefits policy to include only exempt, corporate office employees. Is there any discrimination involved in creating a policy such as this?

No, there isn't. It would be illegal discrimination if health benefits were offered only to male employees, or only to employees under 40, or only to Catholic employees, since gender, age and religion are protected characteristics under the law. But there is no illegal discrimination in offering benefits to exempt employees but not non-exempt, or to only corporate employees but not field office employees, because being non-exempt or being in the field office is not a characteristic protected by law.