Whether or not the WT had to outwardly support the UN Charter is NOT the point.
I would disagree. The fact that they did do that with the UN/DPI scandal in the 1990s was very significant. That truly was hypocrisy of the highest order.
The point is that they went to a meeting sponsored by the United Nations, with UN food, UN speakers, and UN topics....with a United Nation's agenda being dolled out.
If I were a JW, I'd be less bothered by this. Say that an entity of the US or the UK was having meetings on religious freedom. I could accept JWs going to such a meeting as representatives of the Watchtower Society. Yet, the current JW teaching is that the US and the UK are the "false prophet" of Revelation chapter 13. JW leaders have lobbied Congressional meetings before, such as meetings regarding the FCC in the 1930s. JW lawyers have used the US judicial system to try to get their way on various issues. In Canada (arguably part of the "Anglo-American World Power" and thus part of the "False Prophet" of Rev. 13), JWs distributed petitions for the enactment of a Bill of Rights for Canadians back in the early 50s.
Now, if the JW representatives are getting member status in any of these groups or making statements that are supportive of the UN -- that would really bother me as a JW.