[ykevinator]

Its divisive because it attempts to dismiss as invalid the notion that one racial group suffers a disproportionate targeting by police. It denies that racism is a problem or at best equates the problems faced by police to the problems created by police violence toward black people. When you say all lives matter, you're basically looking at these millions of people who are working toward making the world less hostile toward black people and saying "you're stupid, you're the racists."