In the USA it is expected every person will stand during the national anthem, and any other gesture/behavior is regarded as disrespectful. This lies in the country's history and the collective long ago being told what they must do to show allegiance to nation and flag.
People sacrificed their lives and limbs for you and respect for our flag and anthem honors that. Kneeling is plain and simple disrespect for everything this nation stands for. Just like the anarchists burning the flag, I would have trouble keeping myself from getting into a physical fight with those animals and I am getting close to 80.
Taking a knee is viewed as disrespecting your own land. I understand why they did it but they could have done their protest off the field and not during a time when we should be honoring the freedoms we have thanks to the men and women who served this country.