First, the federal police are not letting these cities burn just to spite leftists. Violently ending any riot/protest has historically been a bad move, no matter how justified the police were in doing so. Almost every historical instance of it is almost always titled as a "massacre", and the police know this. Shit, even in American revolutionary history think of the "Boston Massacre". It began over an argument caused by a 13 year old shop assistant falsely accusing a British captain of not settling a debt to a shoemaker, and the 7 soldiers were surrounded by an angry mob of 400 people, literally taunting them to fire and pelting them with clubs and thrown shit. Did firing upon the crowd disperse them? Yep. But it also galvanized the revolutionaries, and the fact that the event is historically referred to as the "Boston Massacre" is pretty self evident point.
Next, I want to give some perspective on the Trump hysteria. I'm not saying that Trump is perfect, nor that Trump is beyond reproach. I have my own criticisms, but the hysteria against Trump is fucking insane and disproportionately huge. For some context, I live outside the US, in a city that's not affected in any real way by anything Trump does. And I work in an industry that is 100% not affected by America. In this past week two insane political scandals/events just happened, one of which deeply involves the leader of my country.
This Friday, my coworkers/bosses opened the morning meeting shitting on Trump for being 'dumb' because he 'bragged' about the difficulty of some test he took. Never once have they brought up my countries politics (despite these massive political events in my country this week), or mentioned any aspect of US politics that isn't just "Orange Man Bad". And outside of the Trump sperging, these are all fairly reasonable, pleasant, and intelligent people. The media/social media hysteria of "Orange Man Bad" is so intense that people in a different country unaffected by his decisions spout "Orange Man Bad" just from their only real exposure to US politics and events being MSM. I never saw any behavior like this from people here during Obama, Bush (father or son), or Clinton. So, you have to ask yourself: "Are Trump's actions so egregious that they justify the level of outrage on a social and media level?".
As an outside observer, my answer is no, they are not. If Trump was truly "part of the problem" in these riots, he would have sent in the tanks and Tienanmen Squared CHAZ. In a revolution, if you want to protect your people the worst thing you can do is give the revolutionaries martyrs. Forcibly ending these riots would only provoke more unrest. And the Federal government knows that too, which is why they're making the smart move of just bottling the riots and letting them either fizzle out, or the rioters start taking actions that will justify full force (armed insurrection, terrorist bombings, etc.).
Forcibly shutting down riots/protests does very little to stop a movement. Dogs, firehoses, and nightsticks didn't stop the civil rights movement, police and construction workers beating the fuck out of protesters didn't stop the anti-vietnam war movement, and given China's insane level of monitoring, censorship, and oppression of their own people, even rolling tanks in and gunning all the protesters down didn't help stop the unrest. It only fuels it. The only way to win is not to play.