I wonder if that was a manufacturing flaw, then -- something like that shell being bored slightly oversize, allowing the outer race of that bearing to rotate when it shouldn't.
I should say that I'm making an educated guess here. I haven't taken apart one of these exact transmissions before or seen this exact kind of bearing wear, at least not in a transmission context. I have taken apart other transmissions, and typically the outer races of bearings are meant to stay put while the inner race and the shaft it is mounted to rotate.
What's strange to me is how there seems to be an oil passage and groove around the outside of the bearing. Is that for oil flow around the bearing or into the bearing? I'm not sure.
Which model is this? I think I might get a clue from the Lubrication chapter of the factory service manual, if the one I've read (for 79-82 DOHC bikes) applies here.