Libel is one of those funny things that is very, very hard to prove, because most courts insist on a *knowledge* of the libel before looking at finding the defendant in violation of libel laws.
In libel per se, the media outlet pretty much has to know what they are printing is a lie when they print it. That's how the tabloids get caught and have to pay up when they are sued. Or, the plaintiff has to prove that the media outlet deliberately neglected to check a "fact" before printing it.
Even so, libel per se is easier to prove than libel per qod. Damage to a reputation is notoriously difficult to prove, and there's still the whole "deliberate" thing.
Reputable media outlets are always concerned about libel because, even if the outlet prevails in the lawsuit, they have spent a great deal of money to prove their innocence. Many outlets will settle out of court, because it's cheaper than a protracted court battle.