The U.S. had a substantial investment in the Entente war effort through huge loans and contracts to provide small arms, munitions and other material. That, apart from anything else, would tend to moderate any response.
There is also, as has been noted, the rather dramatic difference between unrestricted submarine warfare and the internationally accepted tactic of "stop and inspect". One results in piles of dead civilians regardless of what the vessel is carrying (or, critically, not carrying), the other usually results in a brief delay and either authorization to proceed or instruction to either return to point of origin or to a port controlled by the intervening power.
Failure to follow these orders would result in a vessel being disabled (a fairly easy matter for a surface warship against a merchant vessel), the passengers and crew allowed to move to a position of safety (normally the lifeboats), and the ship then either being taken as prize or sunk depneding on local circumstances. The warship is then responsible for ensuring that the passengers and crew are in a safe condition before departure (in practice this meant either towing the lifeboats to a safe harbor or taking them aboard the warship until they could be deposited at a port, either belonging to a neutral power or a friendly port). This entire process had been an accepted part of war for a century or more. It wasn't necessarily LIKED by neutral nations, but it was acknowledged as proper.
Unfortunately, this rather civilized method was utterly impossible for a submarine to follow, hence the term "unrestricted warfare". The practice of "sink without warning" of civilian vessels was (and still is actually) uncomfortably close to piracy, especially in the more genteel early years of the 20th Century (although the tactic was still more than a little questionable when employed by both the KM and USN in WW II). Pirate vs. professional naval officer. Who do you think will get condemned?