These 18 titles were the canary in the coal mine showing that under current German law, it is infeasible to operate a Project Gutenberg-like service accessible in Germany.
It is absolutely possible to operate a Project Gutenberg-like service accessible in Germany. It would just have to serve books in the German public domain, just like the original PG serves books in the US public domain. Project Gutenberg Australia (http://gutenberg.net.au/), for example, has books that are in the Australian public domain but are still under copyright in the US.
It happens that in this case the books are in US public domain but not the German, but that's more of an exception than a rule: current US and EU copyright terms are the same (life of author + 70 years). Australia on the other hand has life of author + 60 years.
The lock-out is just the natural result of that.