I just got home from photographing an event this evening. It was a meet and greet with a local U.S. Congressman.
Mostly stand-ups and handshakes with muckety-mucks but I did a lot of grab shots as the opportunity presented.
When the Congressman arrived, I grabbed a couple as he came in the door then, when the way was clear, I introduced myself and told him that I was the event photographer. I said he could call me over if he thought there was something that needed to be photographed but I also said that he could wave me off if necessary. (For instance, if he had a piece of food stuck in his teeth. )
It worked out well because, once people knew who I was and that I wasn't trying to pull anything sleazy, they all relaxed and I was able to get several good shots. I was invited over to get a several group shots and, in one case, somebody asked me to retake a picture because he didn't like the way he looked in the picture. I was glad to do it and even trashed the bad one.
In this case, I was hired to photograph a (semi)private event but, in the case of public events or news gathering, I think a similar principle applies.
If people know who you are, what you are doing and that they can trust you not to act like a snake they'll loosen up and you'll get better results.
I don't think you always have to spell it out for people. Your behavior can convey the same message. Are you hiding behind a potted plant and shooting from the shadows with a telephoto lens or are you interacting with people? In my case, I thought it was prudent because the Congressman didn't know who I was and I was not a person that he would recognize as a regular member of the press.
In the end, I think it works the same for press photography and event photography as it does for portraits and candids. Your behavior conveys more than adherence to a set of rules or code of ethics. Just as a portrait portrait photographer's relationship to his subject can make or break a session a press or event photographer's relationship to the people around him can make all the difference in getting good results.