Because you can manipulate reasons with evidence. Bias is pretty intransigent (at least to comments on social media). So if you can get people to keep the same viewpoint but embrace the reasoning, you at least have a chance to shift the reasoning and get the bias to grudgingly drift along. If you ignore the reasoning and go for the bias, you're stuck.

(Now, if you can befriend the person, avoid provoking their bias strongly, be understanding, do entertaining or meaningful things together...then...that can all shift their bias. But I'm assuming we're not considering that kind of substantial personal intervention.)

I agree with basically everything else you say...my point was only about how strong your point was, not that some point along those lines was valid.

Edit: I reject the labor rights vs Amazon rainforest destruction analogy, because those two rarely in practice actually conflict with each other. I mean, unless it’s labor rights in the Amazon basin. A better analogy would be switching to electric cars to solve global warming vs. everything else to solve global warming — and there I think the criticism is fair: too much attention on electric cars can absolutely prevent faster and better measures. Not sure if this point is important enough to keep discussing, though.

This time, you're adding more description of the context in which these utterances convey the bias and racism that you say they do. The first time around, you didn't really do that.

So, for instance, if there's (another!) police shooting and someone includes a hashtag or statement about "Black Lives Matter!"; and someone else responds "Oh yeah, what about black-on-black violence?"...that's clearly a manifestation of an ugly instinct. I can't envision any realistic scenario where it's not.

On the other hand, if people are discussing police funding levels, and someone says "Defund the police! Black Lives Matter!" Saying, "What about black-on-black violence that claims even more black lives, and which is reduced by extra policing?" is at least plausibly the start of a reasonable good-faith discussion about how best to structure society given the multiple ills that ail it.

That was really all I was getting at. As a rule of thumb, I think your original advice was on-target. But I think it has the potential to cut off positive, worthwhile discussions if applied uniformly. Applied deftly, or as a reminder that one can infer motivation from the context in which a statement is made, the advice is fine.