As your screenshots clearly show, the angle of view is very different for each resolution at the same zoom level.
...Oh really? The widescreen shot shows more area at the sides than the non-widescreen, exactly as one would expect. And the angles don't seem "very different" at all to me. After examining them in Photoshop, they don't seem to differ by more than a degree or so.
As for the whole "we've gotta stay objective" bollocks: does one rate the quality of a car purely by their horsepower, 0-100 accelleration times, fuel consumption, number of seats, the volume of their boot, and their maximum cornering angles at a given low and medium speeds?
Although there are some things which all good widescreen games ought to have (such as widescreen resolutions being selectable in-game, proper UI scaling, proper aspect ratios), the most important thing, whether or not the game is better in widescreen or not, is slightly subjective.
I think anyone who has actually played Supreme Commander will immediately conclude that it's better in widescreen. The viewing area in widescreen
IS larger and the aspect ratio is correct. Who gives a s*** whether or not the camera angle differs by one degree in different resolutions? If such an utterly unimportant variable results in a "C" rather than an "A", then I think the rating system is pointless.
I personally believe that the rating system would be a lot more useful to WSGF viewers if it weren't to extremely authoritarian. I don't think letting Human's judgement rate a game's widescreen implementation will break the system. On the contrary, it'll make it actually meaningful. Please, go and actually play Supreme Commander - if after playing it you still think it deserves a "C", then I don't know what's wrong with you.