If I were king, this issue would have been solved already. However, I'm not king, and I (like many other people) choose to live under the laws of this country.
Because the Supreme Court said that flag burning or other desecration of the flag is protected under the 1st amendment, in the incident where the American flag was lowered, abused, and replaced with a Mexican flag, options are unfortunately quite limited. Even if the police wanted to intervene (note that I said wanted, because they could if they had adequate orders), they would be left with crimes such as theft, or similar crimes that would in all probability be misdemeanors.
Police departments are stuck with decision making over how much work they want to create for themselves. They have to make decisions about whether it is worth risking the safety of a number of officers to wade into an crowd of hostile antifa's to make an arrest for what would probably be a misdemeanor where no one is being physically assaulted.
Hell, they won't even arrest people for assault in Portland, so I really doubt they will risk the safety of the officers to make a misdemeanor arrest or 10.
I note that in Tacoma yesterday, the police did shoot and kill an anarchist who was throwing Molotov cocktails toward the ICE facility. So I guess it all depends on the crime, and the orders from the higher ups. Perhaps it was because the guy in Tacoma was attacking a Federal Facility and they had orders that were different from those that Portland officers receive. Or maybe it was because the molotov coctails he was throwing would constitute deadly force... In any case, sometimes the police do take care of business.