hate to tell you this but most of those movies are NOT public domain.
Just because a movie is old, unknown, indy...or just plain crappy...it's NOT public domain.
The only stuff on that site that could be considered public domain are the old silent movies...or anything they might have that was produced by the government.
Rule of thumb:
for any creative work, whether in music, art, print, video, photos, etc...
if the government made it, it belongs to 'we, the people' and can't be copyrighted.
If it's from before 1920, it's copyright has run out and can't be renewed, and is now public domain. (unless it is trademarked, like most of the Disney things are...that will never run out as long as the Disney company still exists)