“There was no warning issued”
The Germans took out a full page ad in the NYT saying they were going to sink it because it carried war cargo. The explicitly said they would and warned people not to sail on it.
Nope. The ship was attacked without warning.
A propaganda ad saying “all allied ships”, furthermore, is a declaration of war and a declaration of intent to flout the Cruiser Rules. To agree with the German Empire’s POV is to disagree with the sovereignty of anyone else, including the USA.
I might add that the technique the Reich used by publishing ads in the NYT is akin to what Islamic terrorists do today—advertise their intent and carry it out no matter how reprehensible or abominable and no matter how many international agreements it violates (the latter has social media at its disposal to foment that).
Not to mention, the Ottoman Empire (Islamic Caliphate) was a major member of the Central Powers.