Lewis: I’m not a lawyer but I am not worried about the ethical dimensions of this. How does a human learn to kern a font? Initially and unconsciously, by looking at lots of well-kerned fonts and learning from them the relationships between shapes. Every time a kern a sans /A/V, I am “processing” my memories of Helvetica or Univers or whatever. That’s just how the discipline works.
Ben: I’m not sure why you say that this ignores the spacing. The spacing data is there in the sidebearing contours - the contours represent how far the shape is away from the edge. A contour of [50,75,100,100,...] and one of [25,50,75,75,..] both represent curves into the center but one has tighter spacing than the other. So I think the network is getting this information about the glyph shapes and their spacing together.