The Oath Keepers are a network of current and former military and police that exists to defend Constitutional rights against government overreach. There are a small number of them -- four or five according to reports -- who have deployed to Ferguson on the anniversary of the riots there. They are going among the protest groups lawfully armed. Police may have a duty that requires them to be in opposition to the protesters in terms of controlling violence and lawlessness, but in this way some of the police can show that off-duty they have an equal concern for the rights of black citizens. Although the police leadership has suggested it would prefer they go away, on the ground officers seem to have worked out an understanding with them that's keeping both sides cooler than they might otherwise be.
Naturally, this is unacceptable. It's important to remember that armed white people are scary and unwelcome. Anyone who suggests that the rights being defended first came to be realized because of similar armed citizens -- many of whom happened to be white -- are dangerous history nuts.