And "this must have been caused by racism" sounds like motivated reasoning to me!

If, instead, police 1. treat people equally when encountered 2. encounter demographics in proportion to the crimes committed by that demographic and 3. african americans commit a significantly higher number of crimes per capita... There wouldn't be a way to make the police less racist - since they already treat people equally - but police reform could be effective at reducing the brutality encountered by everyone. Making things a police racism problem (african americans might commit more crimes due to racism from groups other than the police) then just makes it so that no progress can be made on the "police beating people up and killing them" problem, because the police aren't racist.

The above might not be true! But it's believed by a lot of people, and has a fair bit of academic support. (for instance, there are studies showing that police officers were faster to shoot a white man than an african american man when they were taking the same actions - the opposite of what I'd expect if police treatment differences were driven by racism)

I was quite convinced by studies I have seen, mostly those collected by Radley Balko that this your first paragraph is not the case.

But here, we are arguing about opinions on what is going on and not by "you should understand this person can not accept racism as explanation because he would not be benefiting from it".

There, I was asked to understand that reasoning on itself, if it is not benefiting me it can not be true, which is completely different then this.

I'm saying that there's a lot of reasons that someone might respond badly to being told that the problem is racism. One is that the problem genuinely might not be racism, as I've tried to say earlier in this thread. Another is that systemic racism is a society-wide thing, and so by blaming a problem on it you indirectly blame the person you're talking to. (or at least they perceive it as such, even though systemic racism is supposed to be the idea that the system can be racist despite the individuals not being racist) A third is pattern-matching past instances of people saying "the problem is racism" to unpleasant/ineffective outcomes for them - for instance, implicit bias training (which while IIRC there is evidence in favor of implicit bias being a thing, training actually working to reduce it is on far shakier foundations). I'm sure there are more, and not all of them are "racist conservative refuses to accept racism is the problem because they don't benefit from solving racism".

On the other hand, "the problem is racism" is very morally easy/acceptable in leftish circles, and saying the problem is anything else often gets a negative response (this thread existing might be counted as evidence here) - providing motivation to call things racism regardless of the truth of the matter... (This is not necessarily the case, of course - just trying to show that motivated reasoning goes both ways)

But the guy in original story I responded to had one specific concrete reason - if it is racism then solution does not help me. And I was supposed to find that understandable.

I was respousing to that particular logic of that particular guy who was much less reasonable then he believed himself. I was not responding to all the possible reasons of all the people in the world.