My Airbnb host used to "hire" a Japanese kid to water her garden without paying him a cent. She told him it was an "internship" for her design company, and she told him by tradition and by default, internship is unpaid. That kid was paying very high rent to her already, obeying all kinds of house rules like "shower should be less than 5 minutes and only one time per day". He was nice enough to work for her for free for over a month. I don't think that kid knew about his right. That's where the laws should come in.

Thats not the rule of thumb. The rule of thumb is that if they're doing "real" work they need to be paid. In software that generally means anything that goes into a production service or release.