The problem being that even if it was filmed in public, a majority of the people in the area were from the convention and doing silly stuff like this already. Because of all the other people from the convention doing similar things, it really degrades the value of the joke to the people in the area.

Edit: My complaint really isn't that he knows the people, it's just that since there was a convention going on (presumably near where it was filmed) there's a bunch of people already in costume and doing similar things, which partly ruins it.