This question is raised often and debated much amongst lawyers focusing their practice on employment-based immigration. I have a call scheduled with a corporate client who is considering the legality of accepting a volunteer in their for-profit IT business.
I intend to inform them that under US immigration laws, if the work is performed for NO remuneration or other benefits, it would not violate the law. This issue has been explored in my blog entry here.
The problem, however, is that the Fair Labor Standards Act (Federal Law) does not permit for-profit employers to hire unpaid "interns" or "volunteers." See this link for FLSA standards according to US Department of Labor. There has been considerable litigation on this issue with employers on the losing side. So, please consult your employment law counsel before deciding on retaining the services of unpaid employees.