I think you're mistaken. Just because those movies are being sold cheap doesn't mean they're in the public domain.
You really need to research something before you discuss it. I didn't say ALL cheap movies were public domain, did I? You can find a good listing of public domain movies at archive.org Also, I'm pretty sure that any old movie on a DVD has been restored in some way and as such is a derivative work and I couldn't just copy that and redistribute it.
Nope, most have NOT been restored in any way and are rarely taken from the original. Many are taken from old tapes that were sent to TV stations and some were actually recorded from TV. The Three Stooges for example are almost surely still copyrighted even though they started in 1925.
Some of their stuff is Public Domain. Even some episodes of Andy Griffith and Beverly Hillbillies are in the public domain.Anyway, this isn't the thread to discuss your misconceptions of copyright law.