Why does Unity rotate my bones around their centers?

I’ve got a series of turrets that I wish to animate via bones. As mechanical constructs, these turrets have moving parts that only rotate on a fixed axis and around a specific point, so I carefully placed each bone at the desired center of rotation:

Turret with Bones in Blender

Testing the bones in Blender, everything behaves perfectly: the turret can rotate on its base, its weapon arms on their anchors and the gun barrels around their centers.

If I now import this model into Unity, it is put together correctly, but the center of rotation for the bones changes:

Turret with wrongly rotated bone in Unity

Each bone seems to rotate around its own center. In the image above, I tried to lower the right weapon arm, but instead of it rotating around its intended joint, the entire arm rotated in place.

What am I doing wrong?

In the upper left portion of the Editor window, just to the right of the ‘move/rotate/scale’ buttons, is the word ‘Center’ on a button. Click it. Changes to ‘Pivot’ Now try.