It's not just a matter of shifting time, it's a matter of who owns what when. Ownership is and always will be 9/10 of the law, you're not allowed to modify that which you do not own, and within the distribution structure of copyrighted material the store doesn't own it while they're trying to sell it (they own the physical copy, but not the copyrighted material on it). It's not screwed up, it's just complicated.
I think this response makes the most sense of anything I've read so far. Thanks for hanging in there with me. :-)
So, it would seem that the same store could, in theory, have a video sales department selling unmodified copies of movies, and an editing department where new owners of those movies could submit them for an editing service, and avoid legal issues.