sobernow, the plane that crashed in Pennsylvania was, allegedly, going for the White House. It was exactly that - an attack on America. The other stuff was peripheral.
I know what Jbr means by saying that a similar incident in, eg, China, wouldn't have got the same coverage, but the point about Sept 11 was that it was so stealthy and so unfair. Even the IRA generally gives warnings about bombs etc (I believe there was one for Omagh).
So many of those who died on Sept 11 were "workers" - cleaners, waiters, posties etc. And we can relate more to others from a western civilisation, who speak the same language as we do, so we identify with them more.
For what it's worth (not much I know) for a few months in 1981 I worked on the north side of the 79th floor of WTC1, around where the first plane hit, in an office in a rental suite, and I know for a fact that that suite was one of the places that nobody came down from. It's unlikely that anybody there when I was was there on Sept 11, but I know how being there felt, and I identify with the ones who were there like mad.
I won't be observing any silences either but I don't think that, just because it's America, we shouldn't sympathise with the victims and the bereaved.