It is a lazy, worthless protest for someone like Kaepernick not to stand for the anthem. It takes no effort to not stand. He isn't sacrificing anything except possibly his career, although some have speculated that this may actually help solidify his spot on the 49er's since were the team to let him go, there'd probably be considerable push-back, basing his release from the team as stemming from this display (whether or not this is true). No, the US is not perfect, but it is by far the best country for anyone (even minorities) to get ahead in life; in fact a majority of the countries in the world make certain minorities illegal (and, for example, may outright kill you for being a minority like a member of the LGBT community). And given his name recognition and wealth, there are other things that he could have done to truly help the plight of minorities in this country.
When Jackie Robinson chose not to stand, he was already a civil right's icon. His mere presence in the major league of baseball was a stand against the oppression that he felt. He lived during a time when racial strife was so much worse than today. Part of the racism that the black community were fighting during that time was the systemic racism of the Jim Crow laws. Not only were these terrible for their racism, but what made them worse was that it was actual legislation that minorities were fighting. Businesses were forced by law to segregate, whether or not the owner of that business felt that that segregation was wrong. People needed to resort to civil disobedience in order to get their voices heard. And if / when they were jailed because they dared to go against an unjust law, they gladly accepted it as being like a martyr dying for their cause.
I don't deny his right to refuse to stand for the national anthem. I don't think that he should have to leave the country or have to pay a fine (even one imposed by the NFL / 49er's). I just think that he could have done so much more if he truly wanted to do something for the minority community. For example, heed the call from police chiefs throughout the country that say that the best way to reduce individual racism among the police is to actively recruit more minorities into the force; although he may not have given up playing football to become a policeman (unlike another football player that did give up his NFL career in order to do what he claimed was his moral duty -- and in the end, Pat Tillman died for that decision to enroll in the Army after 9/11 attacks), he could have certainly started a trust to provide scholarships to minorities wanting to work in law enforcement.