There is actually very little legislation that addresses unpaid internships. Unpaid internships are sort of an evolutionary byproduct of the need for low wage labor and the supply of young labor.

Neither DHS nor USCIS has issued any guidance on unpaid internships. The closest thing is when they talk about employment for students on F1 visas. In that case, F1 students are required to obtain an EAD before accepting any type of employment, paid or unpaid. However, some unpaid internships may not be considered as "employment", in which case there is no requirement to obtain an EAD.

The closest federal legislation that deals with unpaid internships is the Fair Labor Standards Act, which was enacted in 1934 but subsequently amended many times.

The Fair Labor Standards Act outlines six conditions under which an intern is considered a "trainee" and therefore does not have to be paid. The two most important rules are that an intern cannot do the same work as a regular employee, and that the employer must not obtain "immediate advantage from the activities of the student."

Theoretically, an employer who makes the intern do all the copying and filing is actually violating the FLSA because the intern is doing the same work as a paid employee and that the employer is deriving an immediate benefit, namely diverting paid employees to do other tasks as opposed to copying and filing. However, its practically impossible to enforce these FLSA provisions.

In this sense, Congressional internships would be considered a violation of the FLSA.