Honestly, I think the 2 main components are disrespecting the flag (and by extension America) and declaring America a racist country. You're always going to invoke the deep resentment of the flag waving patriotic crowd with the former, and a lot more people are sick and damn tired of being called racist. And there is a lot of overlap in those groups.
The young, entitled, half-black, multi-millionaire celebrity whining about being oppressed is always going to turn people off, but guys get away with that all the time. The wholesale slander of the country generally took this to another level. That he dragged it onto the football field hits the fuktard trifecta.
On a slightly different note, I'm all for freedom of speech and I think demanding firings, organizing boycotts, and going after sponsors is grimy. I call it cultural fascism. But this is different. This isn't a guy who posted something on twitter, made a controversial statement in an interview, or got drunk and dropped a slur at a concert. This guy went on the field, in uniform, at game time and did this. Now I still wouldn't threaten to boycott a team that signed him, but I don't blame teams for avoiding him like the plague. He did this to himself.
This country has a deep history of racism against blacks. Slavery, Jim Crow, segregation - how can you argue otherwise??? Unfortunately, police treatment of blacks has long been an instrument of that oppression. While there are complications related to crime incidences and gangs, there has still been enough high profile evidence to call for more analysis and discussion, which is all Kapernick was doing.
Honest, open discussions about race, including silent protests, are an important part of our continued national healing from our dark past.
fwiw, Kaep is a pretty bad QB and that's why he's not signed (like Fitz, Geno, etc...)