I find the two sides of this issue are different arguments. One side says kneeling is a constitutional right. The other side says it's unpatriotic. Maybe it could for some be called unpatriotic free speech?
I can see the argument that the NFL players are on work time and so the focus is football. However, some political speech at work has been deemed constitutionally legal, like wearing a button for a candidate, as long as it does not disrupt the workflow. Others on this side will say the owners can stop this, but they aren't. Thus Trump's claim they are afraid of the players who make money for the owners.
Others say kneeling IS patriotic, the freedom to do so being what the Founders fought for. 
I can't understand why freedom of speech is OK but not if it is disgraceful to our nation. That is the cost of freedom. 
No one is saying pick up a rock and destroy the country. If we're going to salute the flag at a sport event (why sports?), then those who dislike kneeling can boo later.