Doctoral dissertations are regarded as substantial, original and new contribution to an area
of scientific study and research and hence their publiction is obligatory -at least at
the unis where I graduated from with a doctorate: To be handed over your doctorate
graduation document, you had to prove that 150 published copies of your doctoral
dissertation were handed over to the university library. To my knowledge, the
publication of Master theses is not obligatory - nowhere.

I am aware that the publication of Master theses is not obligatory, but I was not aware that Doctoral dissertations are actually OBLIGED to be published. An ex-colleague of mine recently graduated with a PhD in Linguistics, and he had chosen the Chinese Miao language as the subject of his dissertation. Having spent several months living with these people in a countryside community, and taking a total of 7 years to complete the dissertation, I guess one might be slightly annoyed if such a work wasn't published.