Some funny posts here. For those who seem unsure, slavery is an involuntary thing. So regardless of what Bjorn is or isn't paying, you always have the choice to simply not volunteer. This isn't slavery - hard stop. I've had many unpaid internships throughout college and grad school. They taught me little if anything and I did it for no reason other than resume building. It is what it is. Choose a path in life and follow it. Earning our stripes isn't evil. If you want to talk about shit work, speak to some plumbing apprentices.
That said, this old-school type of education is counterproductive, IMHO. Much of it boils down to "I had to do it, so you do too". Doctors coming up 40 years ago pulled double all-night surgery shifts. That's not good for anyone, but their mentors had to do it, so now it was their turn. Same for lawyers. Same for soldiers.
I would imagine, however, that the benefit to people like Bjorn in having apprentices is far different than lawyers, for example. Training a lawyer is an investment the firm makes, because if the lawyer performs well this is an asset the firm wants to keep. I'm not sure how many people spend the rest of their careers or any substantial time with Bjorn after interning for him. Rather, they move on and become the competition. What he gets is free labor in return for an education that cannot be received everywhere. Hell, he pays just as much as we earn trying to learn bonsai on this group.