ShareAPIController.php has the comment:
For some reason, single files share are forbidden to have the delete permission
which further indicate this is by design, provided the context is the same. The reasoning must be apparent elsewhere.
I’m still perplexed that I’m not seeing any documentation to support this nor that no one else is observing or commenting on this behavior. Also it’s unclear why the delete option would be grayed out, and not just removed entirely.
Perhaps it’s just obvious the delete option wouldn’t be available for a regular file for the reason I previously stated, though the code comment above suggests otherwise.
Will hold off a little longer. Would like to discover something more definitive.