The phrase "51st state" can be used in a positive sense, meaning that a region or territory is so aligned, supportive, and conducive with the United States, that it is like a U.S. state.
What reasoning would cause a country to be referred to as the "51st state" in a positive way?
a region or territory is so aligned, supportive, and conducive with the United States, that it is like a U.S. state