Regarding the German law
First, there is academic freedom, yes. It's in the 5th article, number 3 (Art. 5, Abs. 3 GG) of the german constitution, the Grundgesetz:
(3) Kunst und Wissenschaft, Forschung und Lehre sind frei. Die Freiheit der Lehre entbindet nicht von der Treue zur Verfassung.
Arts and sciences, research and teaching shall be free. The freedom of teaching shall not release any person from allegiance to the constitution.
Although only teaching is mentioned as bounded by allegiance to the constitution, the systematics of the Grundgesetz make clear that all these rights find their boundings in the first four articles if they collide. They can be found in english on this site.
Therefore, scholars and especially university professors (teachers and public officials) can of course be charged e.g. for denying holocaust, because they violate higher valued rights (i.e. human dignity, art.1 no. 1 and the moral law, art. 2 no. 1).
The reason for explicitly mentioning arts and sciences is for creating rights of their own that cannot be constrained in any way by general law or any other governmental action, unlike freedom of speech (see article 5 nos. 1 and 2). That means the only way to constrain these rights (but by higher valued rights, see above) is making a new constitution!
In this sense, the freedom of speech is extended (less restricted) for arts and sciences (as well as press in a wide understanding as "media", see Art. 5 I, sentence 3: "There shall be no censorship"), because they are more violative to governmental actions as they are supposed to express the whole range of opinions in public, even if the government may not like it.
Regarding extensions of freedom of speech
In a general understanding, any extension (or stronger restriction) of freedom of speech seems injust, because it contradicts any sense of equality. On the other hand, explicitely protecting public institutions like media, arts and scientific institutions seems inevatible. Look at e.g. Turkey (or Russia) what happens if you do not.
So as you can see, you should never equate equality with justice/fairness per se. In fact inequality has to happen if it proves to be better for society. In philosophy, this is manifested in e.g. Rawls' Second principle from Justice as Fairness.
Sorry if this answer is too technical and less philosophical than it could be, I studied german law for two years in Germany and misunderstandings like these are still taken personally ;)