Actually both are true. It is certainly possible any particular act might not be meant in a disrespectful way, and that our interpretation of it is based on as an arbitrary social convention.
However, the point is that such conventions exist in every society and you cannot wish them out of existence. The well-established social convention in the US is that standing for the national anthem is the proper way to show respect. Despite all your words, to the vast majority of Americans, kneeling when you should be standing smacks of dishonoring their country.
Further it is completely obvious that this mode of protest was chosen precisely for its shock value, precisely because it would violate the usual practice.
What you are not allowing yourself to see is that people know when they are being disrespected and this is one of those cases. Suppose a barista greets you with a big middle finger. Even after they explain it is meant in a positive friendly way, I’m guessing you won’t be ordering any latte there.
If the kneelers actually cared about their cause, they would not discredit it by making it synonymous with disrespect to the flag. I notice you didn’t respond to the picture I included of Martin Luther King marching for Civil Rights on the Selma bridge surrounded by American flags. That was a much better way to advance a cause.
The players acts of disrespect for the flag at a sporting event have nothing to do with the First Amendment. The players are free to sit, kneel, or stand on their own time. They accepted big money knowing they would be expected to participate respectfully in flag ceremonies. Then they pull stunts that would get most people fired on the spot. A majority of fans make a lot less and don’t have a lot of sympathy for the whining pampered millionaires.
In contrast, look at every other sport. At a MLB baseball game last night, players from Caribbean islands, Central America, Japan, and many parts of the United States all stood respectfully for the US national anthem. The flag ceremonies at the start of recent NBA finals games were all incredible light and sound shows with great upbeat performances. The players in all the other sports understand that antagonizing American fans by insulting the American flag is not the way to sell tickets. If a football player can’t see that, he is perfectly free to tear up his contract and start a new career as a political activist.