Also Shawn of the dead is a parody and copyright has a special section to allow parody/satire of other works if it's clear they are not the original work but are a parody of it.
To sum up I think more than a failure on copyright notice it was a failure in BRANDING. Branding requires special names.
Terminator can be easily protected but if the movie was called Rising of the robots it would have not been possible to protect it.
BRANDING is an important way to create a moat of defence around the product. Others can copy the product but not the branding
Yes indeed is very difficult to protect names without trademark them. That's why many startups/companies invent completely new names (ex Google). Once you trademark them it's easier to protecte them than a word that was already existing in the English language like terminator.
