Kind of off-topic, but If I take a photo of a Superman statue, am I allowed to sell that photo for profit to a for-profit entity (say a magazine or tv show)? Does D.C./Warner have any rights in that situation?
Yeah, you can bet a corporate entity like NBCU is going to clear its broadcast photography - WHO gave permission is another story, but it may fall on who signed the release forms.
I've never shot any photos like that, but I'm fairly sure editorial use would be in the clear, that is assuming they have put the statue on display and made it available to the public. If you see something at for example SDCC, my bet is that they have signed an agreement when getting their booth stating that commercial photography of any materials on display is allowed.
Basic rule, unless you are explicitly told otherwise verbally or via a "No photography" sign, you would be relatively safe to take photos and make commercial use of them, at least in an editorial context, such as a magazine article on a convention. Other commercial use would probably be riskier, but if anything you'd likely get hit with a cease-and-desist as damages would be very hard to prove. The licencee would have to prove that your specific commercial exploitation of the photos of their statue affected their sales of the statue... now that is very tough for that specific example, but obviously much easier if it was pages of a comic or other graphic material (even if presented in an editorial context).
If the cosplayers signed a model-release that was written by someone who has any clue what they were doing (i.e. one clearly specifying that the photographer is sole originator of the copyrights, which cannot be signed away in any way under US law, and that the model/subject has limited, non-transferrable use for self-promotion in reasonable channels) and the cosplayer, who are most likely working as independent contractors for the show, then gave NBCU those photos under a contract stating that any materials provided were cleared in terms of copyright... then the cosplayer alone is 100% at fault from a legal standpoint.
There are currently no legal requirements for NBCU or its subsidiaries to verify that information, which means that this type of thing happens all the time. Usually the threat of up-front legal costs high enough to bankrupt most small-businesses mean that most such cases never go anywhere, if they are lucky the company won't fight the cease-and-desist and stop using the offending material, but there are no guarantees.
Edit: I also have to agree with Art, the statement the photographers made is not very professional. Posting the correspondence publicly is never a great way to behave in a legal situation. Unless they really want to spend tens of thousands of dollars fighting this, which could easily take years, they would have had more success by finding a copyright attorney to go over the process in detail. Once they had made damn sure that their contract was bulletproof, they could go ahead and make a short, public statement clarifying the copyright details of the case and their intent to recoup their losses... then have their attorney get in touch with NBCU to ask for fair compensation, or be forced to seek damages.