I do think that a travel warning is sufficient enough for you to cancel the trip without penalty.

You can refer to this article for reference: Reiserücktritt bei Reisewarnung: In diesem Fall können Sie kostenfrei stornieren - FOCUS online

Travel warnings are not that rare (Aktuelle Reisewarnungen - Auswärtiges Amt) - I count 43. Admittedly most of them are only partial warnings. There is even one for Japan - "do not go to posted nuclear contamination areas". Okay, do we really need a warning for that? Some of these warnings are lightweight.

I am sure the airlines take these warnings into consideration when they make their decisions, but also use other data sources in their risk assessment.

However, if you are really concerned, consulting a lawyer would be the best bet.