Seeing as that the visitors in the stands also didn't participate this mostly seems to be a misunderstanding where they weren't aware they were going to do this.
Besides, and this will probably not be a popular sentiment, but I'm not a fan of making foreigners participate in our public rituals of mourning. Especially not foreigners from a region where death and destruction at the hands of terrorists is far more common. Those victims rarely get a passing thought over here so it just reinforces how much more important Western victims are thought to be to everyone and not just ourselves.
Besides, and this will probably not be a popular sentiment, but I'm not a fan of making foreigners participate in our public rituals of mourning. Especially not foreigners from a region where death and destruction at the hands of terrorists is far more common. Those victims rarely get a passing thought over here so it just reinforces how much more important Western victims are thought to be to everyone and not just ourselves.