ilovewaterslides said:

Yeah, because in the U.S it's actually illegal to own a 35mm copy of a copyrighted movie.

That's not exactly true. Or, at least it's misleading.

There's nothing inherently illegal about owning a 35mm print of a movie in the U.S. What's important to know is how 35mm films were released in the U.S., at least since the 80's (probably earlier as well, but I imagine someone can correct me if need be): the movie studios would rent the movies to theaters on a weekly basis, and the theaters agreed to a contract that told them how many times they could show the film, how many weeks they could show it, etc. At the end of the run the theaters were required to send the prints back. For a big release, the studios would then have a large (maybe a couple of thousand even) number of film prints with varying degrees of damage that they no longer needed, and the prints would be destroyed.

So, say for example you get your hands on a 35mm copy of Back to the Future from the 1985 U.S. theatrical release, there are probably two main places it could have come from:

1. Someone at the movie theater decided they wanted the print, took it for themselves and reported to the studio that it was missing or stolen (probably couldn't get away with this very often though otherwise you'd never get any more movies)

2. The person charged with destroying the extra prints grabbed a copy to keep or sell

So, this print of Back to the Future would then be considered stolen property, and if you bought it, that's why it would be illegal.

But at this point studios aren't even making 35mm prints anymore, so I'd be really surprised if anyone's looking for old theatrical release prints in private collector's hands.