It is literally NOT illegal. There are no laws censoring certain words on television in the United States, free air or otherwise. It is against FCC regulations and you can be fined though. All of the laws and rulings have to do with the FCC being able to enforce these regulations and levy fines for breaking them.
"It is a violation of federal law to air obscene programming at any time. It is also a violation of federal law to broadcast indecent or profane programming during certain hours."[0]
" In addition, a federal district court may impose fines and/or imprisonment for up to two years on those who are convicted of criminal violations of the law."[0] (Emphasis mine)
