I'm afraid you are missing the point. Fallout 3 offers people WITHOUT widescreen monitors (and people running Fallout 3 in windowed mode) widescreen resolutions that are letterboxed.
Then if 4:3 users won't get an expanded 16:9 frame as a result of the letterboxing, why have black bars at all? Why not just have 2D mode run in fullscreen, and let the black bars take over in hor + 3D mode where this actually results in a 16:9 frame?
Because for a menu you generally want to read it, and, frankly, there is not much extra to see.
For run and gun in first person, you want an expanded field of view.
Now ... there is the argument that you could go into the .ini file and adjust the field of view settings, but ...
a)some consider editing the .ini files in ways that give one a game advantage akin to cheating... and a wider FOV *IS* a game advantage. Having an option in resolutions that allows it explicitly demonstrates it is a regular game option, not a modding tweak or console command (keep in mind what else console commands do).
b)some don't know how to mess with .ini files and frankly would be a menace to their computer if they tried. On a similar note, scaling of both the ver- menus and the hor+ FOV can be done by said .ini file...
... so however you WANT the menus to crop or add, you can do so, whatever mode you are on, widescreen or otherwise. This means for power users, the argument has become a bit of a moot point, unless you change to resolutions with different aspect ratios regularly.
Edit:To have fullscreen for 2D menus and widescreen for the main game engine would involve switching resolutions on the fly every time you pick a lock, which would be a disaster... that or not actually rendering it in widescreen at all, just expanding the FOV and blacking out the top and bottom (which is significantly harder on a video card). Also, many monitors align differently in different resolutions and refresh rates, sometimes shifting the image edge off the screen several inches, as well as having a short time black between. This does not happen with all resolutions, but it happens enough to be an issue on enough systems. Trying to fast switch resolutions to show more dead space on menus is, regrettably, very technically messy.