2014 Ferguson Riots - What Tumblr isn't spreading like wildfire

I'd guess it's because 'cop hugs protestor' is less attention grabbing than 'cop shoots/tazes/beats protestor', plus cops are expected to be friendly (or at least civil) so when a cop abuses his/her authority it breaks the trust people have in law enforcement and elicits a stronger reaction.

Because this is what a cop is expected to be like.