I can reproduce the issue you see there using the Camera interface, and am not sure at the moment why that doesn't work. However, there are several alternate methods that may be more useful that are constructed specifically for off axis projections.
The first is the off_axis_projection function:
A while back this was wrapped with an OffAxisProjectionPlot Plot-window interface, which can been seen just below the example linked to above.
Finally, you could use the ProjectionCamera class, which inherits a lot of characteristics from the Camera class. Here's an example on how to run it (modified version of your code above):
Note that the off_axis_projection function actually just wraps the ProjectionCamera class, and OffAxisProjectionPlot wraps off_axis_projection, so it's all the same underneath.
Let us know if that works. I'd suggest you go with the OffAxisProjectionPlot interface since it is meant to be similar to the SlicePlot/ProjectionPlot.