They call it that in honor of the poor guys who are in the audience against their will.
I haven't seen the movie yet (even though the Aivan says he's *grimace* willing to come with me), and I haven't seen the play either. I've loved the songs since I was in elementary, though, when a classmate described in great detail the scene where Eponine sings A Little Fall of Rain. I practically cried, I was so impressionable.
To be completely honest, I've gotten a copy of the movie from, er, somewhere, and I've watched the opening scenes. Poor Russell Crowe; it's not his fault he'd had to sing in his upper register. I almost expected him to snarl, "Are you not entertained?!" Hugh Jackman looked like a real hobo and sang pretty well. When the Bishop admits him to the church, I had to stop from screaming "Colm!" like a fangirl. Colm looked like he was smiling during part of his song, though, which felt a little weird. And Anne was so pretty in the factory scene, there was no way she could have continued to work there without inciting spite. I stopped at Lovely Ladies because I knew I Dreamed A Dream was coming up and thought it deserved to be viewed in the big screen.
I've never seen a sung-through film, and even I am secretly afraid I won't like it, so I have absolute sympathy for all the guys who are getting dragged to the movie theater. But, hey, at least you'll know he's a keeper if he goes with you to watch Les Miz even though he'd really rather have a root canal. He must (probably) really love you, which is cool, since [a guy in love] is better than an opera. :-P
In the meantime, here's an ode, if you will, to Les Miz by two broke girls in Paris.