Because a huge number of black people in America describe a frequency and quality of interactions with police that are vastly different from white people's described experiences, claims which are backed up by horrifying anecdotes (like this story, or the seemingly weekly story of some other small town police department that was "shocked" to find it had officers in the local KKK chapter) and larger scale statistics on law enforcement.
That is a strong prior. Coupled with the utter stupidity of testing bird shit on the outside of a car to see if it's cocaine, it seems entirely fair to conclude this was motivated by some amount of racism.
As you may have noticed, the trouble with the word racism is that nobody feels they are racist. Nobody feels like they are consciously participating in the behaviors described in books/movies from a prior time as being part of intolerable indiscretions. All while the threshold for getting statistics for an individual event is impossible, yet the demands for sensitivity and societal change are numerous, imperceptible, seemingly random and difficult to understand if they are valid until broader consensus is reached.
Martin Luther King didn't have a statistics or spreadsheets or studies either, but we still accept the shared experiences he brought to the national spotlight as "racism". The things experienced then in Alabama were normalized "what? you think I hate black people? I've never consciously put a single negative thought in my head" behaviors as well. Behaviors that were often economically rationalized, as much as a yellow taxi driver in NYC today not wanting to drive to Brooklyn and assuming black customers in Manhattan want that and decides not to stop. There is a shared worse experience amongst people with similar phenotypes that runs parallel to the user experience people expect and commend.
Here, the disparity of the interactions people experience and expect with police merely continue to confirm what people already believed.
It is accurate that there may be more consistent socioeconomic drivers to this experience, than merely race-based ones, or socioeconomic drivers which correlated to race-based experiences.
But when people highlight "racism", they want a consistent experience in line with their trust in the fabric of society. They want to not experience total derailment and marginalization of their life in a random pitfall that their whole family has experienced in one way or another. Its not "assuming racism", it is just recognizing that this is the colloquialism for understanding how institutions will marginalize people. Does it accurately reflect the experience that many white people are also subjected to when they can't/don't procure good legal representation? Probably not. Fixing accountability problems with the state will help them too.