It's possible you're right about their concerns. But IMHO those concerns wouldn't be valid. At the very least, they probably could assert a copyright claim on the exact board layout, if not the schematic, to fend off competition. It would be legal for someone to clean-room reverse engineer the board (something that wouldn't be all that difficult) and provide a replacement part, so if there's really a big enough market for that to support that kind of effort, which I doubt, it would be happening anyway.

But frankly, surely having accessible replacement parts, whether sold by them or someone else, is better for their bottom line than having their pin sales decline because of concerns about reliability and parts availability and cost.

Now, again...that's not saying there's not some bean-counter at Stern who doesn't see the big picture, and who is resisting the urge to make public important technical documents. But it'd be a pretty short-sighted attitude if so.