The panel orientation property tells compositors about the orientation that displays are built into specific devices. In order to support these devices with rotated displays best, offset the rotation the user can set by the rotation set by the property. With that, normal rotation set in display settings will be what the user assumes to be normal.
If the user has adjusted their configuration to rotate the display already, this will break their configuration again. However, only having the default orientation be set by the property would also be confusing for the user, so I think one-time breakage is acceptable.