Property rights. The right to decide the what, who, and hows involving your property (your body is your property).
Also, the reporters can be there, since it was a public meeting which is implied consent for everybody to attend. That consent lasts up until the point they are told to leave, which they weren't (like how the reporter informs the touchy feely guy to stop touching him). By giving notice, you make it easier to prove intent in court if they don't comply, gain cause for certain actions like calling the police, etc. And a lack of notice is evidence of continued consent. They were neither told to leave nor that cameras weren't allowed. There's consent in their favor all over that video, and no evidence of trespassing. It'd be like advertising an open house in the newspaper plus your front yard, not stopping people from walking through your open front door, then claiming you can sue those who show up for trespassing. You won't win that. The only way you win that is by asking someone to leave, and if they don't leave, then you got them.
And you're right, those people aren't answerable to them. Which is why they have the right to completely ignore every question he asked and generally act like he doesn't exist. Thing is, they didn't exercise it and chose to answer him. Being an investigative reporter and based on the video, he acts like he knows how to avoid a lawsuit which would have made the above possible for them to achieve (he never intentionally impeded the woman's progress or touched anyone, other than the one time he defended himself, announced consent, kept their 2nd camera on touchy feely guy/reporter for evidence against any claims, etc).