It feels like NextDoor is late to the moderation party. As Facebook and Twitter are finding themselves somewhat responsible for what users spew, NextDoor will see the actions like those called out by the top commenter happen much quicker.
As with any destructive force, rioting is not a sustainable state of being. It is a blunt object to signal "Things are not okay" from societies whose frustration boils over in trying to achieve change through more articulate language.
Rioting rarely affects change in the systems in the direction desired, as its methods are misaligned with desirable, sustainable values. Thus, it allows ruling classes to paint a harsh narrative of the rioters - leading in many cases to greater inequality and worse conditions.
Riot theory seems like an interesting starting point to understand the socioeconomic climate in America today. A dialogue from which would naturally lend itself to survey the options that members of a community have in articulating opinions and criticisms of the systems they live within.
Note: I haven't had the chance yet to read the articles linked by Mr Havercroft, but look forward to doing so.