From reading everyone’s responses - it’s fairly clear the answer is no - it’s not illegal to block someone’s private filming of a public event. Nor should it be IMO.
I was watching the fireworks recently, and all the jerks “filming” the event, blocked the view of the public quite rudely, and there was nothing to be done about it.
That said - we can have ALOT OF FUN with the law here.
For example - a person could get a permit from the city for filming, for the date and time of a protest, and that may give them special rights and privileges on city property to film. Including having the police escort someone away from “filming areas” or quite frankly, the right to hire a private security guard or firm to assist, if you have a defined area you can film from.
Further - there is a fun “real estate” argument to have fun with. Federal, State, City, Private lands could provide all kinds of legal powers and loop holes depending on how they could be used. I love the Vertical Easement discussion above - I deal with that kind of stuff in my line of work; and whilst the law is generally in place for permanent structures, one could have fun applying.
LAST - Protesting is one thing, but SIGNS can be something else entirely. I recall working in a computer shop and having to renew permits for having road sign advertising constantly. Depending on city/region, one could argue that whilst the protestors have the right to protest, their signage does not have a valid permit and could be “confiscated”.
Of course, some of these ideas put municipal/federal employees in poor positions, just to have one person exert their rights over another, when simply bringing a LADDER next time might solve the issue. Some morality to consider.
LOL this question gives me a contrasting last thought, if I stood still on a street as a mob of protestors passed, and then I couldn’t leave the area because the protestor mob had essentially surrounded me. Have I been ILLEGALLY DETAINED? KIDNAPPED even? My freedom of mobility restricted? Who gets sued? Individuals? the organizations behind the protest? lolol…
And yes - you do have the right to stand in a movie theatre; or be loud and disruptive there; and they have even right to kick you off their property for doing so.