The rule is: you can't, in any way, compel citizens of other countries living outside your country to do
anything. You really shouldn't even try, unless it's with the cooperation with the other country (just fill a lawsuit there!).
If you do try, it's a straight path to extraordinary renditions, drone strikes, assassinations and other such joyous things. I'm not saying that it has to lead to this, but it could, it did in the past, and we've instituted a rule to prevent that. Please, honor the rules we've all agreed upon!
Now, back to the issue in question. If you put a site on the Internet, you're by default giving access to it to everyone - you need to put some effort into it to make filtering traffic possible. It doesn't matter how much effort is needed - just that you have to do something.
This court order tries to compel PG to do that "something". It doesn't matter, at all, how many books are involved. It doesn't matter that it's "just for German IPs". It doesn't matter how easy it is to implement. What's important is that the court tries to compel citizens of another country to do something. No matter how small it is, it's simply wrong. It shouldn't happen!
The court could force ISPs in its country to filter traffic so that you can't access those books. It could make filtering these books to be hardcoded in home routers and public access points. It could, for all I care, ban the Internet in its entirety - inside your country. It cannot, and should not even try, to force PG to do anything.
Now, there are exceptions to everything, established precedents, international agreements - I'm not a lawyer, I don't know if there's a legal argument which would allow the interference in other country affairs in this case. What I'm saying is that such interference, as a rule, is not allowed, supporting it should be viewed with suspicion and every argument presented should be very carefully scrutinized and verified.
Again, as a rule, you don't force foreigners who never set foot on your soil to do anything. Infringing on another country sovereignty is simply a no-go (unless you have a lot of nukes, apparently, but I digress). Why is it hard to understand?