Yes, they are there, so Big Brother is watching you watch. More or less, it wasn't a problem until people started to record the movies and put them on the internet, which really takes away from where movie theaters make the most profits, which would be concessions.
I don't think they are trying to find Jim and Jane making out in the last row, but are there to try and stop people from making pirate copies of certain movies.
Have you ever wondered how come some movies are on the internet in a heartbeat, so to speak? It's because the people who are recording them are uploading the bootleg copies of the movie, not even waiting for the DVD release which should be a much better copy.