Here is some cut and paste from the laws of The United States:
Unpaid Work is Work – Make Sure You Have the Correct Visa
to distinguish whether volunteer work is unauthorized, it is helpful to understand the government’s motives in setting up this regime. The USCIS’s goal is to prevent foreign nationals who have employment restrictions in engaging in work which could give them an advantage over U.S. citizens or which could drive down Americans’ wages or benefits. Therefore, it is not permitted to volunteer for a productive position which is usually paid. The rationale is that if you are offering to work without a pay in a job which would otherwise be filled by an American who would be paid, then you are subverting the employment authorization system’s goals and undercutting Americans’ job prospects and wages.
