The nice screenshot, here:
Very unwieldy image
tells us that absolutely nothing in the base game is classified as "Structure".
If I switch the setting to "Other", only the struc is lathed.
I'm not going to believe this is intended.
Also a meta-request, a forum isn't a great bug tracker, Virgil, could you please post an "Acknowledged" below threads that you've read and will work on? Otherwise I keep asking myself if I should repost things because you may not have seen them.