No one is told to bow their heads. The people who are praying may be bowing their heads and most have their eyes closed and cannot see anyone who chooses to not participate.Even though some refuse to comply, all are told to bow their heads. Even the "you know it's a myth" billboard still only presents an idea for consideration.
As for the billboard, I can only assume it is just a matter of time till I get a knock on my door from a proseltizing Atheist. :ignore: