It's got to do with the legal distinction between a font and a typeface. Fonts being the digital implementation of an original typeface design.
The Akzidenz-Grotesk typeface may be over a hundred years old and out of copyright, but the font that happens to share its name and is obviously based on it, is a new and distinct legal object, subject to its own copyright period.
You're (probably) within your legal rights to create a brand new font that also happens to be slavishly based on the original typeface, as long as you create it from scratch and don't use the name 'Akzidenz-Grotesk' (hence, fonts like MS's Book Antiqua being a blatant ripoff of Palatino and other similar cases).
There are many versions of Times, Garamond, Caslon and other typefaces sold by different foundries. Each implementation is copyright to the publishing foundry, even though the name is the same. The design is in the public domain; the implementation is not.