I happen to believe the police went in there to essentially just standby as security. There is effectually a mob on their property - and while they appear to be respectful and keeping an appropriate buffer, all it takes is a few people to unleash the potential energy of a large crowd. Trust me, I have seen my fair share of 'mosh pits' very quickly turn into the 'wall of death' formation really quickly! If you're prepared for it, it can be great fun! If you're not, it can get REALLY out of control, and my point is, there is never any warning. Anyway, whether a metal show, or a front lawn, human psychology is the same - it just takes one person to start chaos.