I believe it wouldn't even be legal for an unpaid intern to be used for "actual work", as they're supposed to strictly be involved in mentoring/training activities. Hopefully Square knows that, and wouldn't try to treat its employees unfairly in the first place. They seem like a great company.

Programming internships are typically paid positions. I can't speak for Square, but every other programming internship I'm aware of (even at other startups like Path) were paid.

Yes, we do pay our interns.

Legal or not, it's all too common outside of the tech industry, especially in the creative industry.

Square pays their interns.

Definitely getting paid. Square treats us interns well. :)