In this case, it's the administrator of the Facebook group "Bernie Sanders 2016 -- Ideas Welcome! (sic)" She's announced that she's deleting all posts about a planned rally in DC on 28 November, and may ban anyone who tries to post about it.
I hadn't heard of this and know nothing outside of what she says about it. She argues that it's poorly planned and won't get any media attention unless something unsavory happens there.
In fact, by banning such posts, all she's guaranteeing is that they'll appear elsewhere instead where she can't see and respond to them.
As I find such censorship intolerable and refuse to be party to it, I have left the group.