I've covered this before. If you were referring to Morro Castle, you would have similar circumstances. Havana was the major port. Guantanamo bay was not.
Guns of Castle Morrow overlooking Havana Harbor.
Apart from that, we had the Fort by Treaty with a foreign government. In a divorce, people keep the property that was theirs before the marriage.
Said nothing about Moro Castle. Guantanmo Bay is the only protected harbor in Cuba. Better than Havana. Our naval base there prevents the current Cuban Government from using the harbor for their economic interests. Why shouldn’t they fire on it and cause it’s garrison to surrender. After all it is their property. The land upon which Fort Sumter was built was granted in perpetuity to the Federal Government by the State of South Carolina Legislature. Just because the government changed does not automatically abrogate the pact made by both parties years earlier. Just as the change in the Cuban government does not abrogate the treaty both parties signed years before.
You truly are that naive, aren't you? Holding to that "logic" Fort Sumter belonged to the United States government before the "divorce" and remained their property afterward. Of course there was a short period where it was stolen from them, but fortunately it was recovered.