A Jehovahs witness is still respectful to the country they live in, and other countries as well. That is why we pay our taxes, follow the laws etc.
We will even stand respectfully when the flag is presented, so far as the act of standing, in that particular circumstance, is not itself the act of worship. This is in obedience to the scriptures that plainly say that God does not share his worship with images representing anything in the sky, on the ground, or in the sea.-(Exodus 20:4)
So if an announcement is made "everyone please sing the national anthem", then we could stand. But if the announcement is to "please stand and sing" or even just "please stand for our national anthem" in most cases a JW will not stand.
Not because we are being disrespectful, but because in that instance, standing is what is being considered the act of worship.
So it all depends on the situation, and some may be confused or not hear the wording used and so they may stay seated just in case.
I think as long as they aren't getting up and leaving, it shouldn't bother anyone. At least they are respectfully sitting quietly instead of continuing whatever it is they were doing, now that might be rude....
Just my opinion
Oh and I do realize that in the situation that the OP is referring to, those people may not have been JW's. But I'm just throwing this out there as something to think of before immediately judging someone...