German companies operating in the US need to respect US law. As the article noted:
"Q: Why did this all take place in the German Court system, rather than the US - where Plaintiff does business as Macmillan, and PGLAF is based?
A: The legal guidance PGLAF received is that US law requires that such proceedings would have taken place in the US, and in fact any attempts at enforcement of the judgement would need to occur in the US Court system. PGLAF already informed Plaintiff and the German Court that the US Court system is the appropriate venue for Plaintiff's concerns. Plaintiff declined."
Also, German courts need to respect international law and treaties. As the article noted:
"Alternatively, international treaties - notably the Berne Convention and related treaties - provide mediation processes through the World Intellectual Property Organization. PGLAF offered to undergo this mediation process, and Plaintiff declined.
International treaties explicitly and unambiguously support PGLAF's legal guidance as described above: that the copyright status in one country is not impacted or enforceable or otherwise relevant in other countries. Plaintiff managed to find a German Court, and some precedents from Germany (and, after the lawsuit was filed, from the EU), which were willing to flaunt international treaties by developing a theory that PGLAF is under jurisdiction of the German Court system."