I heard this with a different twist.... The story was that John Denver had a concert, and before it started he asked everyone in the audience who was a JW to please stand up. There were several there, and after they stood up, John Denver asked them to leave.
I've heard it told that way, and I've also heard it told that John Denver was very patriotic, and he would ask EVERYONE to stand for the national anthem. Those who didn't were asked to leave. Of course JWs, always looking for persecution, took it to be against them personally.
Whenever I went to a sporting event, I would stand for the National Anthem, or else conveniently be out getting refreshments until it was over. Cut me some slack though, I didn't join up voluntarily.