Funnily enough, marking content that's harmless as only for adults was also punishable, though that might have been in context of a different law. That would be censorship, blocking people under 18 from accessing legal content, was the reasoning. Welcome to German bureaucracy.

But you are right. It's an argument that the "just mark content accordingly" is also not a better solution, not that ID requirements are in any way better. The only solution is not to enable this censorship infrastructure, because no matter which way it's done, it will always function as one.