These are software engineering internships, where top firms pay interns around $10,000 a month. Ethics aside, it wouldn't be in a company's best interest to offer unpaid internships because they wouldn't get any good candidates.
Source ?
Numbers probably aren't including housing, which in Silicon Valley puts a ~$7500 monthly salary roughly up to $10k.
Meanwhile, the top HFs and prop traders will pay the same or better (often higher pay, no housing). Jane Street is a big one (esp. if you go to Brown), Citadel isn’t even top-notch and will offer that, and then obviously D.E. Shaw, Renaissance, Two Sigma, and so forth.