Who said its for free? Nothing is for free. Flight instructor internship pays $20 per hour for both domestic and international students, and they can work for 18 months or 1500 hours, whichever comes first. Max allowable flight time per month, in accordance with F-1 visa restrictions, is 80 hours. So, $20x80=$1600 per months.
Ferry pilot internship is paid per trip, approx $50 per flight hour. A regular ferry trip from US to, let's say, UK is around 35 hours in a light aircraft. So, $50x35=$1750. Pilots can do 2 trips per month, on average. No restrictions by visa, as all flights depart US with intent to arrive at an overseas destination. Pilots can do this on business visa.
So, not exactly free, is it?...