In a perfect world, you wouldn't have to show a scene like that in the trailer. But that's not the world we live in. I equate it to McDonald's. People go to the movies, and they like to know exactly what they're going to see before they see it.
Nobody wants to be surprised anymore.
I ranted a bit, yes. I hate trailers that give away too much plot. The objective is to sell tickets, not provide the best possible entertainment experience. But after I'd calmed down, I had to admit that Zemeckis was sort of right. We're so trained to previews that it actually makes some people uncomfortable to go into a film without knowing anything about it beforehand.