Comment 1 for bug 304445

Here's the relevant entry from the X.org man page:
 BusID "bus-id"
              This specifies the bus location of the graphics card. For
              PCI/AGP cards, the bus-id string has the form
              PCI:bus:device:function (e.g., “PCI:1:0:0” might be appropriate
              for an AGP card). This field is usually optional in single-head
              configurations when using the primary graphics card. In multi-
              head configurations, or when using a secondary graphics card in
              a single-head configuration, this entry is mandatory.

Put simply it's not a bug but the way it works. Maybe we should mark it as wishlist? (I would like this to be fixed too)