Because lying itself isn't against the law. Lying under certain circumstances, such as under oath in a court or fraudulently representing yourself in a business deal, is. The specific court case OP is referring to had to do with whistleblower status of two local Fox affiliate reporters who were fired for refusing to voluntarily redact claims made in an expose on rGBH hormones in cow milk. A replacement report was run that countered their claims.
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The court found that the reporters were not eligible to be protected under whistleblower status because it is not against the law to lie on television and therefore they were not whistleblowing any crime.
There are a couple of reasons for this. One is that protection of the freedom of the press is taken very seriously in the USA. A law that created metrics for "truth" of reporting would be abused to silence reporters by government via self-censorship. The other reason the court didn't find the other way is because courts in the USA cannot find someone guilty ex-post-facto to established law, so even if this case made a point about lying on television, the court can't just up and make up a ruling and find Fox guilty. The court case was not about the legality of lying per-se.
This case was a terrible side-effect of legitimate concern for freedom of press in my opinion. Maybe there should be some sort of requirement for truth in reporting, I haven't thought about all the possible side effects or benefits of such legislation, so I'm not saying I think news organizations should be "allowed to lie" or anything like that.