Take all this with a big grain of salt—it’s based on the oddly naïve assumption that the police are trying to catch the actual instigators, and that they need real evidence to get convictions.

In my experience, the objective of the police is to create a particular public narrative, with the least amount of effort or risk to themselves. The narrative (which they present to the media after the fact) is that they acted with restraint, respecting the peoples’ right to assemble, until a handful of agitators turned destructive and the demonstration threatened to escalate into a major riot—at which point they swiftly intervened, caught enough of the agitators to prevent an escalation, and saved (most of) the city’s businesses from destruction.

Now, they do want to intimidate the crowd to keep things from escalating too far, but they also want to allow for some destruction to legitimize their tactics and to support the argument that the police force needs more officers. So they leave the actual instigators alone, because they’re useful to their narrative (up to a point) and because the police don’t want to engage with a group prepared to fight back. (What they really want to avoid is a large crowd seeing the example of multiple people physically resisting the riot police without being immediately subdued.)

Instead, they target:

• Journalists, street medics, and legal observers, to remove the demonstrators’ sense of institutional support and legitimacy;
• Anyone whose mugshots will alienate public support—the homeless, minorities, and anyone whose face is vaguely weird or scary;
• Anyone unable to resist a violent beating (like the disabled, elderly, and children) for pure shock value and crowd intimidation; and
• People who came dressed in black bloc fashion, but are clearly by themselves, passive, and not part of an organized group.

These last are the only ones they will try to prosecute, and often their black bloc attire plus the testimony of cops who claim they saw them engaged in destructive activity will be enough to get a conviction. In this case the anonymity of their dress backfires, because the cops can pin the actions of anyone with similar clothing and body type on them by claiming they saw the act first-hand and caught the suspect immediately afterward.

Meanwhile, the real instigators are convinced that they escaped due to the brilliance of their tactics and not because the cops had no interest in catching them.

That said, all this goes out the window when dealing with Trump’s federal agents: they’re working from different narratives with no pretense of protecting businesses, maintaining local support, or respecting anyone’s rights.