American jingoism is rooted in White Supremacy. I'm not saying you can't be black and patriotic, there's plenty of things I love about this country. But this countries history of "Patriotism" is so deeply rooted in conquest and the oppression of minorities at the hands of White men.
The White man sees himself as the human manifestation of this country. He's America, The Man. So whenever ANYONE(let alone minorities) show disapproval or anger in anything representing America, the White man See's that as an attack on him. So he reacts accordingly. They don't see the kneelings from Black players as "Americans taking a stand and exercising their rights" since they only see themselves(White men) as "REAL Americans"(we've all heard this phrase). They view the players kneeling as, ungratefuls who don't respect the sacrifices, "REAL"(White Americans) made so that those ungrateful kneeling blacks could be free.
They see it as an attack on George Washington, Thomas Jefferson, and all the White forefathers of this nation.
The White man feels that HIS criticisms, complaints and anger of things happening in America, is the ONLY valid complaint, and the only one worth listening to. He wants all of to see things though the White Gaze. He wants to dictate your anger.
Angry at the Government?
Cool.
Angry at the Government because of systemic racism?
Not Cool.
Angry at black on black crime?
Cool.
Angry at shoddy and corrupt Policing in the black community?
Not Cool.
The list goes on...