In a lot of cases, old service manuals are in the public domain -- and therefore free to copy, sell, and distribute. That's why you see many of them on eBay in CD format. Unless a manufacturer keeps the copyright renewed, which many don't on discontinued products, then it becomes public domain. The copy of the '68 shop manual I have says that it's an "Officially Licensed Product," printed in 1996, but the only copyright date to be found is 1967 on a reproduction page. It looks to me like they never renewed the copyright, but instead are just marketing it as a licensed product. I'm not sure what that means, but I don't think that it's copyrighted anymore. I'm not a legal expert, however, so don't hold me to that!
-Winston