I think it's because a lot of people hear "Black Lives Matter" and hear that means that somehow white lives don't matter, or black lives matter more than white lives, when they're really just trying to say "Black Lives ALSO Matter," or "Black Lives Matter TOO." Could some groups perhaps explain this better when it's brought up? Sure, but that's one thing with BLM; I don't really think it has an organized hierarchical structure, or any actual official "spokesperson" from what I've heard, which means that some member/groups can be peaceful, some may not, some may be very good at explaining their views, and some may not, etc.
Of course police shouldn't abuse ANYONE, white, black, blue, green, pink, purple, etc. but people saying police shouldn't abuse black people doesn't mean that they're saying it's OK for them to abuse white people.
Just like if a Jewish group tried to get laws passed to prevent/limit Antisemitism, that doesn't mean that us Jews think that people should be allowed to target/harass Muslims for their faith. People just tend to focus on themselves and the groups they belong to and/or associate with. It's really that simple.