I, too, cannot think of a single legitimate reason why some would need to video-record/camcorder a movie. In fact, I have a hard time find a good illegitimate reason to video-record/camcorder a movie. Also, is this *really a problem? (see tangent below)
(a bit off topic)
I often wonder why the movie industry felt the camcordering was threatening to their business. The only experience I have with camcordered video was when I was a freshman; my roommate had a camcordered version of the Matrix. Us, being uber-nerds, watched it a few times before it was released on DVD even though it (the camcordered version) was the most unwatchable, garbled, bouncy, noisy, miscolored piece of junk ever!!
This version certainly did not stop any of us from buying the DVD; the camcordered version was just that bad. Have these camcordered versions gotten significantly better lately?!
Also, although I have almost zero experience in content piracy, my interactions with hard-core pirates and bootleggers who represent that most egregious infringers leads me to believe that they would never touch camcordered versions of movies because the quality is so bad. A buddy of mine who swaps DMB bootlegs has strict standards about the type and brand of recordable CD he will swap with. I am merely trying to say that the most hardcore infringers probably don't deal in camcordered versions of movies (it’s DVD ripping – thanks DVD Jon).
Beyond that aspect, I think it is unlikely that camcordered versions stop people from going to the movie theater.
Also, it seems to me that majority of the movie-camcorder pirates are probably employees of the theater, so it doesn’t make too much sense to statutorily give theaters power to “detain” the pirates. Think about it, how do you sneak a tripod on a theater (to make a “good” camcordered copy) without being an employee? Does anybody foresee employees tattling on each other, let alone “detaining” each other?
This is yet another law that makes me think that I could be a Congressman, or at the very least, an advisor to the entertainment lobby (yes, I really do have that big of ego).

Great points, Matt. I have heard some stories, perhaps apocryphal, about movie patrons setting up tripods to get cleaner versions. You may be right that most of them are movie theater employees.

As for who wants the crummy camcordered versions of movies, the official line from the DOJ is that crummy camcordered versions from the US get pressed onto DVDs and sold in Asia. How often this occurs isn't well-documented. Unquestionably, many warez traders wouldn't want to be associated with poor camcordered copies; but in the race to be "first," I think even a poor version can make the rounds and, thus, in theory enter the pipeline for overseas piracy. Eric.