Once you paid, they got you. If the movie is bad...then too bad.. we got your money.
Actually, not many people know this, if you dislike a movie you can demand your money back and legally they have to return it to you. When Resident Evil: Extinction came out there was a woman in the theatre who absolutely hated the movie. She stayed through to the end credits and left very angry. When I left the theatre they were refunding her her money for the film ticket. I asked her about it when she left and she basically told anyone who would listen that they would return your money if you complained about it, but you couldn't leave the theatre before making the demand. If you left then there was nothing that could be done.