I agree with Openminded's comments on this. Additionally, the Society may have viewed thie membership as merely using 'unrighteous riches' as they have spoken of in the Watchtower regarding what individual members can do. For example, JWs speak against the governments as instruments of Satan. Yet, JWs are permitted to take advantage of government welfare programs, unemployment compensation, Social Security, federal and military retirement benefits ... all as part of their "rights" under Caesar's laws ... the Society uses the Court system to legally establish their rights, as well as use of Corporate laws in each State.
So their view of this UN membership may well have been justified as merely another 'right' to participate as an NGO to gain needed recognition and assistance in cases where JWs were persecuted in some countries.
The one 'Fly-in-the-ointment' is that the Society uses the 'Library card' excuse when they could have made better justifications as noted above. Also, UN membership is not as essential as other useful rights, such as welfare, Social Security, or unemployment benefits. The Society could still appeal to the UN or to the European Union, or the United States to get needed police-type attention in areas where persecution was a problem. Yet, they took this extra step anyway.
The other 'Fly-in-the-ointment' is their signing on to the UN Mission and Goals ... individual citizens who take advantage of government benefit programs do not have to give such oaths and committments ... rather as long as they paid taxes they are entitled regardless of their political views or lack thereof. Yet, the Society made this major compromise of their teachings ... a compromise that individual memebrs would be sanctioned for doing. That is the real sin of the Society ... and they do not want to take the same discipline as they meet out to the rank and file.