Only because Trump waived sanctions and allowed them to dock.

Responding to reporters' questions on March 29 about whether the ship would be allowed to dock, Trump said, "We don't mind somebody getting a boatload...because they have to survive."

The ship would be allowed to dock regardless of what Trump decided. Just Because Trump gives permission doesn't mean Russia needs permission from the US. If the US boarded the Anatolky Kolodkin, that would be an act of war against Russia. That exactly why the Cuban Missile Crisis was so tense: if the US did stop Soviet ships bound for Cuba then that would be an act of war against the USSR.