There's no justifiable reason for why companies shouldn't have to pay their interns.
Pursuant to the United States' Fair Labor Standards Act (FLSA), an internship can be unpaid if it meets very specific requirements, the most important one being that "the intern's work complements, rather than displaces, the work of paid employees while providing significant educational benefits to the intern." If the wording there seems a bit murky, that's perhaps intentional, considering the amount of companies who benefit from unpaid student labor each semester. The law was actually rewritten earlier this year following a string of class action lawsuits that were leveled against Fox for not paying its interns. The new law considers the seven parameters outlined in the FLSA, described as a "primary beneficiary test," as flexible, with no single factor being determinative. Unlike in years past, no threshold related to these rules has to be met. The law is now far more subjective and overwhelmingly benefits companies who wish to hire interns without paying them.