This isn't about the level of intelligence of the average American. (Why do I have the feeling I've just set up the perfect punchline for someone else).
This is about art and what it means to the culture. To dismiss the opinions of anyone concerning "likes" and "dislikes" because they can't pick out some country on a map seems a little unfair.
However, I do think you raise an interesting question. Is it the prop that's iconic or is it the film that's iconic? I submit that most of the time it's the film. What collectors want is to own, no matter how small, a piece of that film. Look at Kane. The sled by itself is no different than any other sled of it's time. What makes it special is that it was used in a "classic" film. Likewise, with Indy's gun. It's special because it was used by Harrison Ford in a well loved film.
Will it still be valuable 20 years down the road? Who knows. But again, I don't think it matters. For the buyer of this piece, I'm sure, is ecstatic to own his small piece of a modern day Hollywood "classic".
I'm not dismissing them because of their intelligence. I'm talking about that only in regards to you point about what's popular now. In a fairer respect I should have said, "These are the same people who made Paris Hilton a celebrity!" The point being that while any schlubb may know/like Raiders better, any discerning movie connoisseur will agree that the sled from Kane is so much more important to Citizen Kane than the gun that Indy didn't even have in the second film was to Raiders to the point that they're not even comparable which, if you read CTF's post, was to make them appear on the same level. If it was a hat, whip, jacket, hell, even the bag, I'd see it more.