Your interpretation is correct, the intent is that they operate on a purely reactive basis. But it's a show of strength to assuage the public rather than actual

Your interpretation is correct, the intent is that they operate on a purely reactive basis. But it's a show of strength to assuage the public rather than actual security -- if there was an actual security risk then you would most definitely see their rifles loaded properly and the area would be closed off to the public (especially to ambitious photographers). As for him clutching the grip instead of slung over -- he is probably posing for the picture and he's clearly displaying appropriate trigger discipline, so I wouldn't be concerned.