In some fields, the unpaid internship is valuable experience. For example, the legal profession. Legal services organizations, such as your local public defender's office or legal aid society provide more experience in 4 months than a $160k job with a big firm would provide in 3 years. You see actual clients, do meaningful legal research, and even appear in court on a regular basis. In contrast, the cushy firm job sees you in an office performing document review (i.e., looking for grammatical/spelling errors) for close to 2 years with the occasional real assignment every few months.
Also note, that in the legal profession, lawyers do not begin generating their profit for their employer (in a firm setting) until 3-4 years in -- it costs money in lost billing opportunities (clients will not pay for a new lawyer's work whatever the quality, but firms must pay salary).
(The 4 month period is chosen because that is the lag between the Cali bar exam and the release of results in late November, the "post-bar" period during which finding a paid job is close to impossible if you didn't already find one before graduation.).