My understanding is that video black is supplied by a standard resolution DVD player above and below the image information area. Thus your television is receiving a normal NTSC full 4:3 ratio video signal and the DVD player can still use the full 4:3 area for menu and status icons outside of the picture area.

On your television, for an incoming signal from and HD source where the 4:3 ratio is not supplied, likely the television itself fills in with grey to accomodate your 4:3 viewing area.