Actors have no past and no future. They are made up fictional charters. They appear for an event and then disappear into the crowd. They were there during the event. You and I were not there, we only got to hear the news story and we believed it. The media are not allowed on the scene, so the only photos we get to see are the ones released by the people staging the event. The actors are not going to come out and say, "I was just acting."
Even if you believed that the actors wouldn't say anything immediately after the event you have to realize that there are people who know them in their real lives. Family, friends, people that they went to school with and who they work with now who recognized them when they saw them on the TV or the internet. How do you stop them from coming forward and saying; "Hey wait that is not really Kaitlynn Cates", "I live next door to Heather Abbott and she wasn't injured" or "I work with Patrick Downes and he is only acting!"? You make it seem like they cloned these "Actors" in some Secret Government Lab and when they were done with this crisis they melted them down into a puddle of goo to be used again later for the next acting job. These people are real people with a real past and a real future.