I'm not at all defending his actions, but it is a free country and he has the right to not stand up during anthem. NFL has already released a statement that players are encouraged, but not required to stand during anthem.
What disappointments me most is that while I can see he wants to stand for people of color, this isn't the best way to go about it, in my opinion. This creates more angst and not progress. I believe if you want to change things, you have to do from within and attempt to connect with both(the oppressor and the oppressed) and look to create a healthy dialogue between the two. Get in the communities of the oppressed and ask what they need from you and look to help them. Get in the town hall meetings and respectfully say what you need to say. Talk to law enforcement and politicians and those in power and work to seek change by telling them what you and people of color need from them. I'm sure it won't be easy, but those are more positive actions to take, than taking a seat.
This thread seems to have proven your point.