That is by design. The Z AOV uses the far clipping plane for areas where there are no objects.
You could build a custom depth AOV:
- Get shading point in world space (state vector)
- Transform to camera space (space transform)
- Negate/complement to get positive value
- Write to float AOV
But if you have a skydome visible to the camera, you'll still get non-zero values for empty areas.
// Stephen Blair
// Arnold Renderer Support