Believe it or not, I thought Kaepernick was actually making a somewhat valid point when he started kneeling during the anthem at football games. It was a bold move that showed he was willing to risk his career for something he believed in, which I had to respect. And he had something of a point = for example, despite the fact that white and black people use marijuana at basically the same rate, black people get arrested at four times the rate whites do for marijuana possession. In some cities it can be as high as 10-12 times the rate per capita. One of President Nixon's aides even admitted, on video, that the drug war was largely launched with the goal of targeting blacks. 
However, there is no excusing the sheer stupidity of what he is doing now.
Who was Betsy Ross?
Betsy Ross is a minor American folk-hero. Supposedly the creator the first American Flag, she supported the revolutionary war and is said to have presented the flag to George Washington himself for approval. She became especially dedicated to the cause after her husband was killed while serving the Americans. She was also a Quaker who lived in Pennsylvania. The Quakers were overwhelmingly in favor of abolishing slavery (although I couldn't find more specific info on Ross in particular), and Pennsylvania was a free state. The flag she created was to celebrate George Washington's cause and the new nation. It had nothing to do with slavery.
Kaepernick and Nike
None the less, when Nike decided to roll out a new sneaker with the Betsy Ross flag design on it, Colin Kaepernick decided it was a racist throwback that represented a nostalgia for slavery. Of course, there was a huge backlash in response, as Kaepernick had just picked a pointless fight on an issue that nobody cares about. Like many SJWs, he basically took something mundane and decided to run with an incredibly pessimistic interpretation of what happened. Somehow he feels like his interpretation must be right and he just needs to pick this fight for some ungodly reason. Of course, he has now burned a lot of political capital and fewer people will take him seriously in the future, which is really a shame if he ever speaks out about an actual important issue again. That is an "if" though.
Why is the Left Picking This Fight
To be fair to the left, most of us just aren't picking this fight. It seems that Nike basically just folded in the face of Kaepernick's criticisms. Most people on the left recognize that this is totally stupid. Conservatives seem to be trying to paint the entire left as supporting Kaepernick, which is pragmatically a smart move, but isn't entirely honest.
Regardless, let's all fly our flags and have a happy 4th of July!