Then I guess they have a bug, or maybe they thought it wasn't important enough to implement. In either case you should report it to them if it's a problem for you.
Many apps aren't going to support polygons very well, since the triangulation necessary for rendering on graphics hardware is tricky to implement properly. If you want to make Collada models that work well with many different apps, triangles are your best bet.
Sorry I am catching up late in this thread.
Google Earth does not support polygons. This is not the only feature it does not support, for instance I think it does not support vertex color either.
Sketchup export to google earth triangulate all the geometry.
I think there is also a Sketchup export to COLLADA direct, which will produce more advanced geometries (such as polygons) that wont load in Google Earth.
I recommend using the COLLADA Refinery to preprocess your COLLADA files for any given target application. Refinery should have a triangulation conditioner that will take care of that problem automatically for you.
Also, I recommend always using the coherency checker on a COLLADA document you have a problem with, and try to load it with several applications (like FXComposer, Feeling Viewer, COLLADA RT...) to see where the problem is.
Regarding the lack of polygon support for Google Earth, you may want to contact them directly to see if they are going to fix this or not. This is really up to you, and like I said it should not be a problem they do not support it, since you are supposed to condition your data between your DCC tool and the target application, using a COLLADA conditioning pipeline.