If Denmark was noble enough to allow a referendum in a time when it was regarded the correct thing to do, it does not mean that the referendum can be abused so that the U.S. can later annex the substantial natural resources of Greenland and say it is for everyone's security.

You cite legalistic reasons that make it impossible to annex Alaska. I'm sure similar legalistic reasons can be found to reverse Denmark's offer in an age that has turned neo-colonial.