The short answer is, they forgot to put the copyright notice on it, and at the time, that made it public domain.
Nice vid on this topic: https://www.youtube.com/watch?v=UI1kqlt4vkA
Ah, after further research I see that the US only joined the convention in 1988.
It's the first movie adaptation of Richard Matheson's "I Am Legend."
It was filmed in the suburbs of Rome, Italy.
The short answer is, they forgot to put the copyright notice on it, and at the time, that made it public domain.