What if every single one of those additional arrests is a Black person? Well that might require an additional look. It's possible that it's all Black people doing those crimes. It's also possible that the mathematician built an algorithm anchored on years of arrest data from racist cops doing racial profiling.
Will the mathematician be able to say, "hey cops can we take a second look?". No. Can the police be trusted to dig into the nuance themselves? Also no. Is racism active and documented in police departments across the country? Yes.
Should everyone who has any ability to do so stand up and say, we won't support this shit, which is what these mathematicians are doing? I think yes, but that's obviously my opinion.
(1) Statisticians predict where crimes are likely to happen for the police using data.
(2) Police predict where the crimes are likely to happen using highly biased guesswork.
The statisticians are being irresponsible pushing the police towards (2). Option (1) can be de-biased, can be debated, its effects can be assessed and its parameters can be tweaked over time.
This is a choice between two options and the people signing on to this letter are arguing for the worse one. They link a bunch of news articles, but the academic complaint seems to be that the model says "assume crimes happen in high crime areas". That isn't a very scary model, and if anyone has an alternative they should be pushing it towards the police, not holding it back.