I've noticed that too. And hold on to your seats for this one:
First, a disclaimer: I'm not a lawyer. But...
As I understand it, if I am understanding the law correctly, then...
*As long as a film or show is from 1976 or earlier*...
...then, if there is no copyright notice on it, or if the copyright notice is not "legally proper" (meaning including the copyright symbol or the word "copyright" *and* the year *and* the name of the copyright owner)...
...then the film or show *forfeits copyright protection* (whether registered or not) and legally falls into the public domain!
That happened to the 1965 "New 3 Stooges" series (cartoons with the Three Stooges voicing their own characters and doing short live-action intro skits). Those cartoons say "(C) 1965" on them, but they *should* say "(C) 1965 Norman Maurer Productions," and because of that error, their copyrights were invalidated.
This *cannot* be retroactively reversed (for instance, by adding a copyright notice to a reissued version of a show). However, I would definitely consult a lawyer before trying to capitalize on this. I also understand a similar thing happened with the entire first season of "The Abbott & Costello Show." But there was some loophole with that (I'm told), in that the owners were able to correct their oversight as long as they did so within a year prior to the expiration of the original copyright term (28 years).
Other films have become public domain because the copyright owners registered them under a working title (Laurel & Hardy's "Be Big") or never renewed the copyright after its original term expired (Laurel & Hardy's "Flying Deuces," The Three Stooges' "Disorder In The Court," scads of Looney Tunes, nearly all color Max Fleischer cartoons).
Again, I am not a copyright lawyer. Also note that copyright laws changed in 1978 and again many times since then. The above mentioned loopholes would no longer apply to shows being produceed today.
Also, it's worth pointing out that some studios such as Disney have been able to persecute companies which have *legally* released public domain Disney material (believe it or not, there are a few actual, bona fide *public domain* Disney cartoons, including two Mickey Mouse shorts--"The Mad Doctor" and "Minnie's Yoo Hoo"--Donald Duck in "The Spirit Of '43," and two other cartoons, "Hooked Bear" and "Susie The Blue Coupe") and claimed ownership of trademarked characters such as Mickey Mouse or Donald Duck. And Disney won. Similarly, though the Jimmy Stewart film "It's A Wonderful Life" is and always will be public domain, a few years back, Republic Pictures was able to halt unauthorized releases of it because they bought up some elements of it, I don't remember exactly but it may have been the script or the musical score, and on those grounds, they have halted public domain releases of the film. That's why you don't see "It's A Wonderful Life" 50 times a year during the holiday season any more.