“allow such highly charged groups to meet?”
I have heard, unconfirmed, that the police led the Unify Right or whatever people into the area where BLM and ANTIFA were waiting, then withdrew. There are some longish youtube speeches alleging this.
Alex Jones outlined this whole scenario.
A Freeper was there and offered this eyewitness account.
http://www.freerepublic.com/focus/f-news/3576803/posts
Archive: https://archive.is/qO9MS