Why did there seem to be absolutely no security during the World Cup? The Death Eaters seem to have waltzed in, spread terror, burned everything and waltzed back out with only Arthur Weasley opposing them. Hell, the whole crowd seem to have had time to abandon the site before the ministry arrived.

I've seen a lot of answers stating 'they weren't expecting trouble' but come on, thousands of wizards in one place and they're going to behave because we asked them nicely?

Cricket isn't known for its violent fans but there's always police on duty at an international cricket game. Is there ever an explanation for this, say from the books or JK herself?