Just the super weird "degrees are everything" culture. The foundation is, as helge5 mentions, that there are certain qualified professions where the guild should ~~get paid~~ certify everyone who practices. The stupidity is that "programmer" was counted as an "engineer," at least in my case. I see helge5 thinks you are allowed to practice CS without a qualification - I can only tell you that the Finanzamt Köln/Bonn disagreed.

And as the other commenters noted, I _did_ say "selbständig" when I meant "freiberuflich". Can I get away with blaming autocorrect? :)