How to Determine if an Individual Is a Volunteer or an Employee
This article explains the difference between volunteers and employees, as well as listing the 7 factors for determining if an intern can be unpaid or not. (The quick answer is that it's likely cheaper and easier to pay an intern than to legally offer an unpaid internship.)
There are no general regulations that permit volunteering of services to an employer in the private sector. All hours worked must be paid.