After the September 11 terrorist attacks on the USA, some news outlets asked or ordered their on-air personnel not to wear American flag lapel pins, red, white, and blue ribbons, or other patriotic symbols in order to "maintain an image of impartial neutrality" and to lessen the chances that overseas reporters could be harmed by those who might view them as an arm of the American government. Prominent among these outlets was the ABC network:

"ABC News has asked its staff not to wear American flag pins in their lapels, or even red, white and blue ribbons, in an effort to protect their credibility as objective sources.

"We cannot signal through outward symbols how we feel, even if the cause is justified," said ABC spokesman Jeffrey Schneider. "Overseas, it could be perceived that we’re just mouthpieces for the U.S. government, and that can place our journalists in danger."
References :
snopes.com

David Westin noted that “after 9/11, the question came up and we, as a matter of policy at ABC News, tell our people on the air, you shall not wear an American flag or any other symbol on the air.” His explanation for the policy:
“I think our patriotic duty as journalists in the United States is to try to be independent and objective and present the facts to the American people and let them decide all the important things. Now, I respect Tony’s right to wear one. I respect any other news organization taking a different tack, but for me, part of the symbolism of the fact that what we’re doing in our constitutional democracy, what we’re trying to do to help quote, ‘the cause of the country overall,’ is to be objective and give just the straight facts to the American people and let them decide what they want to do about it.”

References :
http://www.mrc.org/cyberalerts/2003/cyb20030120.asp#5