A little background on why some programs have black bars on the left and right side of the picture... and some don't.
All commercial Philadelphia TV stations only use a High Definition DTV encoder for broadcasting 24-hours a day. WHYY has both a Standard Definition and a High Definition encoder. WHYY uses the Standard Definition decoder during the day and is currently retransmitting their Channel 12 signal. Because they are using a Standard Definition encoder, your DTV receiver and/or TV can change the picture to fill the screen. In the evening WHYY is using their High Definition encoder, the same as all the Philadelphia commercial TV stations, and retransmitting PBS's HD programming. Because the commercial TV stations are always using a High Definition encoder, when the program is not HD & 16:9, then the HD encoder upconverts the 4:3 programming so that the HD encoder can broadcast the signal. This upconverting adds the black bars on the left and right side of the screen. It's basically telling your receiver that the programming is HD & 16:9. However, to maintain the 4:3 aspect ratio, the black bars need to be added. In summary, because the commercial stations only use an HD encoder, if the programming is not HD/16:9, then you're going to get black bars on the left & right. And you won't be able to trick your DTV receiver into stretching the picture because it already thinks it's getting a 16:9 picture. If TV stations wanted to, they could stretch the 4:3 picture to 16:9, or any variation there of, before broadcasting through the HD encoder. This could fill your screen, but the aspect ratio would be messed up and not pleasing to watch.
Hoepfully this answers a few questions.