"Proprietary" could mean they are considered a valuable trade secret. This doesn't provide any of the protections of copyright, but could be a liability for anyone who discloses them without permission.
Fonts are the same way: the shape of a letter is a mathematical curve, and is furthermore a utilitarian invention since it is a tool used to convey information. But by presenting them as "font programs" that have "creative components" (like text comments), the vendors attach an aura of copyright to them.