It's not banned, it's just that networks refuse to air it out of respect to the families. It is a rule that the military is the first to notify families. How would you feel if the first news you heard of your child is to see his dead body on TV? Moreover, broadcasting the video just assists Al Jazeera/Iraq in their violation of the Geneva conventions.
They have aired atleast one of the POWs (Hudson) because his mother already knows.
I'm sure some network will eventually air it, but they will take a lot of flak from their audience over it.
There is freedom of the press, and then there is good taste. You will note that after 9/11, no pictures were shown of the shredded human body parts that littered Ground Zero. Why? Was it against the law? No, networks had nothing to gain from grossing out their audience, pissing off the familities, and doing something tasteless.
See alt.binaries.pictures.tasteless on USENET for those. There are some people who like gruesome video, who read medical books to see chopped up humans, but most people would rather read about it, than to see it.
Now many people will say "maybe if people actually see the result of bombs and guns, they will be less in favor of them", and that sterilized casuality reports promote support for war, but I really doubt that is the case, since we got more than our share of this in Korea and Vietnam, but it didn't stop us.