I think that USW might not have been a cassus belli, bur the problem is that it didn't just fall from the sky and can't be handwaved away for no reason. For USW to be removed or delayed significantly Germany needs some other success earlier on to reduce the need for it.
Forced or not, USW was still a German error. The Germans hit the 'I believe' button and removed a known and knowable variable when they considered the case for it.
Honest math in German naval estimations.
Or mathematical literacy by the Army General Staff or Reich Civilian government when the Navy briefed the unrestricted submarine warfare plan.
Look, it was nice of the Navy to propose in USW a solution to Germany's increasingly desperate and gloomy wartime situation, so it made sense for the civilian and military supremos to hear them out. But the audience was too quick to press the "I believe" button before thinking and checking whether the numbers presented really added up.
The Germans saw the negative trends of the war and blockade. They weighed the Navy's solution, USW against shipping, judging universal application of the tactic would sink enough Allied cargo tonnage Britain and/or its armies or factories would starve, thus ending Germany's war & blockade problems at the source.
Great. Go for it. The Germans had the decency/rigor to weigh the counter-argument against it - a decent probability the US would declare war. Their counter was two-fold, that the U-Boats would sink all the troopships (which didn't happen) and, much more reasonable, that America's small army and military potential would in any case take so long to apply to European battlefields that the issue of the war could be decided by then.
But Germany's treatment of the counter-argument was cursory and very incomplete.
It left out a couple of important variables: a) Central Powers shipping interned in US ports - upon American declaration of war, this interned, inactive pool of shipping instantly becomes available for Allied cargo purposes, and needs to be subtracted from any sunken tonnage totals. b ) the most combat capable branch of American armed might - the US navy and its numerous destroyers - unlike the US Army, are available to participate in combat immediately, including directly against submarines and in support of blockade operations.
I've never seen a thing indicating the Germans considered the self-negating effect of USW sinking Allied tonnage, but 're-floating' former CP interned tonnage as Allied tonnage in the event of US entry.