I never figured out the idea of an unpaid internship. I started out at about twice minimum wage for a defense contractor while I was in college. Some of it may have been dull skut work which no one else wanted to do (oh, tape boy! Make a dozen copies of this and send it out to the project distribution list), but quite a bit of it was interesting, important work.
But the same work on an unpaid basis? Nope, I'll take the job down the street.
Engineering internships/co-ops are great, that they are paid is the result of supply and demand. Hard sciences/engineering demand is much higher, compared to the supply, than history/political science/journalism.... I also had a paid engineering summer job after switching majors (and while I went to work in the Congressman’s office expecting not to be paid, they actually had a paid slot available they gave to me), in the right circumstances the opportunity to work for free can provide a tremendous learning opportunity. The problem is the abuse of the “apprentice” situation by those who give meaningless tasks instead of a good mix of more interesting work (committee hearings/research/writing and review of your work with very professional and constructive “criticism”). Fortunately I had very good internships (both paid and unpaid), but I have heard horror stories.