It's not a minor issue. It detracts significantly from the really issue at hand, making it nearly impossible to engage with protestors in a dialog as the small number of folks intent on violence and looting take all of the resources and attention, severely undermining the point of the protests, especially because it provides those disinclined to sympathy for the real issue to paint all with the same brush and so dismiss them.
Before it was looting, it was that kneeling at a football game distracts from the game. Before that, it was "no, no, protest will never work, you need to get more black people elected! Try to get a black president maybe!" Before that, it something else equally absurd.
No matter how black people try to get their voices heard about police violence, there will be people who doggedly tear those voices down by any means necessary.
We're talking about the fact that 14% of the US population can't call the cops for fear of dying.
Kneeling during the national anthem at football games should have been an issue between the players and the NFL, but the kind of people who attend football games disapproved of it.
In Minneapolis today due to the videos which were captured even those anti-kneeling americans largely recognize the problem and support solutions. But by allowing the rump to be violent (or equivalently, serruptitiously distribute bricks and gasoline) the activists are losing public support.
Roughly 200 mostly commercial buildings burned in the main shopping area for (working class) Minneapolis over the weekend. The food and pharmaceutical desert is going to kill far more of the poor and marginalized than police violence would have, in a city that already votes D up and down the ticket.
The violence is not necessary, nor productive, nor are the situations comparable.
Expecting the protesters to somehow control everyone in the protest, when the cops fire indiscriminately on people whether they're peaceful or otherwise, doesn't seem like a working system.