Only in Theaters

Why do movie studios think it's necessary to add "only in theaters" to the end of their previews and promos? They all do it and I really can't figure out why.

Are they afraid people will think it's a TV show and therefore NOT go to the theater to see it? Are they afraid instead people will sit at home surfing from channel to channel trying to find the movie?

Or are they afraid people will go to the store looking for the DVD instead of shelling out $10 to go see it at the theater? Yeah, the studios push DVDs pretty hard when a movie becomes available, but are people really so stupid as to not realize the difference?

I can't think of an alternative reason for adding this stupid phrase to the end of the preview. I know the public can sometimes be awfully dumb - but I think this presumes a lack of intelligence on the part of movie-goers that isn't justified.