For what it’s worth, I started my career as a contractor at Intel and later at McAfee.com when I was in high school. They couldn’t have hired me as a full time employee, but as a contractor I got valuable experience and a decent (for a kid in high school) hourly wage. I didn’t get benefits and I sometimes had to do work nobody else wanted to do, but I think it was worth it. Those jobs gave my career a huge head start and I learned a ton.
Obviously not every contractor is in that situation, and there’s plenty of exploitation going on, but it’s not all bad. I certainly wouldn’t compare it with slavery. I think that comparison is a bit offensive. Slavery was an absolute horror and a crime against humanity, not a case of a voluntary work force being exploited for low wages.