There is another possibility. I had suggested in the past that by using US aircraft to transport Georgian troops to Georgia was a de facto entrance of the US into the war. (In addition, rumor has it that US Forces may have fought alongside the Georgians...) Further, while the Russians didn't attack them, there is the possibility of capture. If the Russians surrounded the Vessel, it could result in the vessel's capture. Moscow would be happy to release it, if the US stayed neutral.