I am located in Augusta, GA and my local NBC and CBS affiliates do not broadcast 5.1 sound.
If your local affilates don't bother to support DD5.1, then there is nothing Comcast can do about it. Comcast just passes their broadcast on to their customers. Comcast does not access NBC's or CBS's signal directly, they get the channel from the affiliate. So if the affiliate does not pass DD5.1 to Comcast, then Comcast can't pass it on to you.
Check your local reception thread to verify that your NBC and CBS affilates do not pass DD5.1 via OTA broadcasts. If they don't, then I suspect that you are out of luck with Comcast and to persue this topic directly with your affilates.