Interesting question. I think it comes down to people's perception of injustice.
I think people believe that the rate at which civilians, specifically African Americans, are being killed at the hands of the police force is reaching the proportions of an endemic injustice.
The key word here is injustice. People are killed all over the world. The Syrian civil war that has been raging for the past 4 years has claimed over 200,000 lives, and yet mass protests about their deaths don't seem to be happening.
Instead, I think the protests about the shootings of civilians like Michael Brown in Ferguson, MO are more about people who feel that too many police officers are getting away with homicide, acting as if they are above the law...
They believe that the shootings that make the news are simply the tip of the iceberg when it comes to the impunity police officers enjoy when it comes to harassment, violence and racism towards the civilians they are paid to protect.
On the other hand, although it is common knowledge that the police are doing a very difficult job, and are always going to be resented and distrusted by certain elements in society... There doesn't seem to be the perception that they are persecuted and targeted to the same extent.
What happened in Hattiesburg was truly vile. But I think people don't see it as a commonplace problem that is being underreported or covered up, as they do with the shootings of black civilians.
I was interested in the statistics behind both these perceptions, and tried to do some research into the topic...
After reading a few articles, I'm surprised to report that the answer is not that clear!
There is a website called the "Officer Down Memorial Page", which documents and reports on officers who are killed in the line of duty. According to https://www.odmp.org/search/year/2014, 127 police officers died at work in 2014 (26 of them were automobile accidents).
However, when it comes to civilians, there is actually no official record. It's down to each police jurisdiction whether they make this information available.
Here's a quote I found on Wikipedia (http://en.wikipedia.org/wiki/List_of_killings_by_law_enforcement_officers_in_the_United_States,_2014)
"Although Congress instructed the Attorney General in 1994 to compile and publish annual statistics on police use of excessive force, this was never carried out, and the FBI does not collect these data either".
I couldn't believe this was the case and, if true, I think it only adds to the perception that there is something the police are covering up.
When looking up estimates that have been published for the year 2014, the figures ranged from 400 to 1300
Surely on matters as serious as civilian being killed, the police must be responsible for publishing some sort of accurate report?!