> people host interns because "it's the right thing to do" for the industry

This is bullshit, assuming that we are talking about for-profit corporations.

In my experience, the reason why my teams have hired interns is to get a solid, multi-week preview of their potential quality and abilities. During the hiring phase for juniors, the signal-to-noise ratio isn't very good, but a good intern can get a lot done in one summer. You can easily pick the best one or two interns to hire when they graduate. Then you dramatically reduced junior engineer hiring risk. Also, if they suck or are disappointing, then the loss is minimal -- don't hire them. If they are really awful, then just throw shit work at them and don't waste time trying to mentor them. Really, it goes both ways.