My guess is that the U.S. told them to allow all ships to pass. Maintaining open passage on the high seas has been one of the foundational principles of U.S. foreign policy for decades. It’s the reason why the U.S. found itself in the unusual position of siding with Egypt and the Soviet Union against Great Britain, France and Israel during the Suez Canal crisis in the 1950s.
The Bosporus and Dardanelles are not exactly the high seas and are quite narrow. In most circumstances they would be considered a part of the country. But with Ottoman Empire on losing side in World War 1 and given the strategic and commercial importance the straights they were viewed differently and subjected to international treaties
https://en.m.wikipedia.org/wiki/Montreux_Convention_Regarding_the_Regime_of_the_Straits
“It’s the reason why the U.S. found itself in the unusual position of siding with Egypt and the Soviet Union against Great Britain, France and Israel during the Suez Canal crisis in the 1950s.”
Which, apropos of nothing at all, just happened to coincide with the Hungarian uprising against the Soviet Union and its Hungarian lackeys, and the subsequent invasion by the Soviets.