Wow.. Why are they allowed to make threats??? When I first read your post quickly, I thought it was a warning from police to the protesters.
I just linked an article discussing protests, it was written in the context of journalists being arrested during the Riots/protest. It also mentions...
http://blogs.wsj.com/law/2014/08/14/ferguson-unrest-focuses-attention-on-rights-of-protesters/
It appears the key word is, violent.
http://www.merriam-webster.com/dictionary/violent
vi·o·lent
adjective \-lənt\
: using or involving the use of physical force to cause harm or damage to someone or something : showing violence
: caused by physical force or violence
: trying to physically attack someone because of anger