In the US, completely unpaid internships are rare. Most are paid, but fairly poorly. There are a few major reasons for this:
You have to meet a lot of requirements for unpaid to be legal, and it all has to be documented.
Internships are a "farm" program- many interns are offered and accept a full time position afterwards. If they were unpaid, they are unlikely to accept.
Minimum wage is an absolute joke everywhere in the country. Why bother fighting it when you can pay as little as $7.25/hour? Even doubling or tripling that makes it appealing to poor college students and the farm program, and won't cost much.
(Your example would be illegal in the US, and possibly even enforced)