This might be the case, but I think that maybe the original request ( Hey guys, what if we could film such an event and use it for our promotion ), translated into legalese, had to be that broad otherwise it made no sense ( ie. that Eventbrite would have had to run after everyone to ask for permission to film, or that after filming, the participants would have to veto it ).

This is what I mean, hey lawyers we want to film events for promo stuff, and lawyers go through all the risks and make the contract super aggressive.