To the best of my knowledge, the difference is that a freelancer can charge a lot more per hour… An intern’s salary on the other hand caps at $15 but its a lot easier to get a foot in the door as an intern.
Other industries don’t restrict internships as a “students only” thing but it also seems unpaid internships are more common in those fields (take journalism for example). Maybe its just an issue of supply and demand. There are more recent grads with comparable ID skills and fewer full time positions?