Skewed Logic-Enrica Lexie

What is the logic behind asking the Italian Government that the ship Enrica Lexie can be released only if the Italian Government can guarantee the presence of the other four marines on board the ship? If the ship is detained in the absence of such a guarantee it is the owners of the ship who suffers. The marines are not private guards employed by the ship's owners and they are directly under the Italian Government. The owners of the ship has no part in the whole affair except that it so happened the marines when the did the alleged shooting was on board the ship. Using a reverse logic, it is like keeping the ship in the custody of Kerala government as a guarantee for the appearance of the other four marines at the time of hearing of the case. If the ship was the property of the Italian Government it has logic. But it is owned by private party. Coming to the comparison of imaginary railway situation, it will be keeping the railway compartment as a guarantee for the presence of witness at the hearing of the case. Can the railway held responsible for the appearance of witness who were passengers at the time of incident?