Why #TakeAKnee upsets people is why antifa upsets people. Bear with me.
The national anthem at the beginning of NFL games means, arguably, that the fight on the field, and the partisanship in the stands, is bracketed within an allegiance to national unity, so LA and Denver don't take their rivalry too far, and don't forget that "We're all Americans." So when Kap sat during the anthem, he was saying, "nope, I'm not one of you -- Black folks in this country aren't 'Americans' and fellow citizens, because they don't enjoy the protection of the law." In other words, Kap was denying the performative myth of the anthem, that "we" are united as Americans. Black Americans are subjects, not citizens, and the American state is a conquering force, not a civitas. (This is why all the "unity" bullshit this last weekend is bullshit. "Unity" is just the national anthem again. Kap was denying that there is unity.)
Antifa make the same move, and signify something analogous: they deny that they are fellow citizens with fascists, capable of tolerating their abhorrent opinions so long as everyone agrees that the state is the arbiter of any disputes that get physical. Liberals like Jonathan Chait consistently think that the Antifa are claiming that "either the government or violent street fighters" have "the right to silence opponents of the left." But this is not the Antifa claim at all. They are claiming that they *are right* to fight white supremacists, not that they (much less the police!) *have a right* to fight white supremacists. They are denying that the state is a legitimate arbiter between opposed groups of citizens making incompatible claims. That doesn't mean they are claiming the mantle of legitimate arbiter, but that they do not think a legitimate arbiter is a possibility in this situation. In other words, like Kap, they are denying that the invocation of corporate unity is capacious enough to encompass them -- so long as it also encompasses people organizing themselves to make the US a white "ethno-state."
Both of these claims are genuinely upsetting if you think the constitutional and legal framework of the US is a legitimate one, one that imposes upon people the duty to work through its established institutions and legal mechanisms. But the claim to legitimacy is also -- and always has been -- a claim to have established the conditions of peace. Whether or not a polity has established the conditions of peace is not a moral question. It must, to some important extent, accept the people *as they are.* If people are not, in fact, pacified, you do not have the conditions of peace. You can argue that people ought to be pacified, but they are not under any sort of prior constraint to accept your argument, and you sure as hell cannot presuppose that they are bound to accept it before you have even offered it.
You want peace? You want people to stand for the anthem? You want people to acknowledge their citizenship in the same country, and under the same laws, as you? Fine. Then make the country and its laws fit for their acknowledgement. Make the law and its agents treat them as citizens. Give them a good reason to lay down their arms in the faith that no asshole who thinks they are less than human will be able to subject them to inhuman treatment. Otherwise, admit that they are a subject population, at war with your polity, and get on with it.