However, if PETA feels that a perfectly legal and well-maintained slaughterhouse should be taped and the 'horrors' shown, that's a different story. Just because they feel 'meat is murder' doesn't mean that it is.
Under 1st Amendment rights, they're allowed to do this. They're not showing anything that didn't happen so it isn't libelous.
Personally I think the first amendment is out of date. The appeal to it's authority is a prime example of the Historians Fallacy[1]. In many other countries there are restrictions on speech -- and that is seen as a generally good thing. Eg. hate speech, and holocaust denial. Defamation cases are also able to hide behind the first amendment in the USA, where they cannot elsewhere.