If you arrest someone for an offense you know they won't be prosecuted for, that's just harassment. It'd be exactly like arresting someone for smoking pot in a jurisdiction where the DA's office has a policy of never pressing pot smoking charges. That's the sort of disproportionately minority-biased policing that even many conservative criminal justice reformers have railed against.
Except in this case the DA adheres to a criminal justice theory that is so absurdly radical that their policy is to, e.g., never press serious charges (i.e. charges that potentially come with incarceration) for property crimes because they believe (not entirely unreasonably) the criminal justice system is irredeemably corrupt, property crimes are less harmful to society than the incarceration norms (not nearly as reasonable), and that by taking such a radical approach he might induce structural change elsewhere in the system (notwithstanding that, like in the parolee case, he doesn't actually try to make sure that change is happening). Moreover, robbing people at gun point is a property crime, not the type of violence exception that warrants prosecution and incarceration. That's Boudin. I've never actually heard a meaningful defense of his decisions. At best his small cadre of supporters seem to rely on their faith in his abstract principles to quietly console themselves, including the wife of the famous private investigator recently killed in the street by thieves, whom he was photographing while they robbed someone else.[1] Everybody else in the city, left, right, and center, is basically just slack-jawed. But he's an elected official with independent powers. The process for removal is well underway. I'll eat a hat if the removal fails.
FWIW, he seems like a very intelligent person with strong convictions, and while too radical for my tastes, I could respect someone who put in the effort and followup required. He seems to have many of the qualities needed for someone to have any chance of succeeding in that regard.[2] But like conservatives who advocate and practice malign neglect in a misguided attempt to subvert and extinguish the administrative state, his approach is indistinguishable from nihilism, and that's indefensible for somebody entrusted with a public office, particularly the DA's office, which at least in principle is only quasi-political.
[1] Of course, since they actually ended up killing someone, Boudin's office is prosecuting them. But when you invite so much risk-free property crime, these sorts of "accidental" physical assaults and murders are inevitable.
[2] Though, his credibility is increasingly suspect as he fumbles to explain his office's decisions and square them with facial claims about prosecuting all crimes. It's clear he's begun playing semantic word games. His office may nominally have a policy of prosecuting all property crimes, but if you only require a fine or restitution and take incarceration off the table for failure to meet terms or even to show up to court at all, especially when people are what civil trial lawyers call "judgment proof", the claim has no substance.