Had Mr Kaepernick started his social injustice stance before walking on the field or after it, nobody would have an issue with it. However, the second he did it at his workplace, his employer and all future employers have the right to terminate and deny him employment. You may not agree with what the White Supremacist/Nationals had to say, but they didn't make their stance at their workplace. They legally obtained permits and held a rally just like many other groups have done.