I'm not as critical of the "anti" crowd as @Fluf is here, but he does correctly identify the side that doesn't like it.
The national anthem playing before every sporting event is just a thing that happens here; a tradition that kind of began back during WW1 with baseball, but not before every game back then. Then in WW2, it became common to play it over the PA system before every game. I imagine the other major sports have simply followed suit, as football, hockey, and basketball were either not nearly as popular or not played as much as baseball back then.
The side that approves and likes players sitting/kneeling (as was far more common last year, and maybe the year before, I can't recall), is that it is a protest of sorts. To stand with the #BlackLivesMatter movement, and raise awareness of the issues with police brutality (particularly, in their opinion, towards minorities, and especially black folks). To the supporters of these causes, these are people using their public imagine to bring awareness to an important issue in American life.
To detractors who dislike these kinds of showings, it seems disrespectful to the nation, and particularly, military both past and present. To many of them, it is one thing to advocate for a cause, but another to actively show disrespect for the flag - something that is meant to symbolize many things that are important to many people. By showing disrespect for the flag, they argue, you also show disrespect to every officer who serves who doesn't engage in brutal behavior; every military serviceman who has ever fought for the freedoms you now enjoy, and so on.
There's also some subset of people (I don't know how big) who simply don't want the politics of the day coming into what is generally seen as a recreational thing; something where we can all put aside the hard issues of the day, and enjoy a competition together without having political messages becoming entwined with the sport.
That's basically the two "sides", and why it's a thing.
i wouldnt quite say that i am anti since i do still stand for it, its just on of those things where everybody is supposed to do the same thing and it looks like brainwashed madness to anyone not used to it.
I don't think it's so extreme as looking like brainwashing, but I can see how it might be a bit of a culture shock for people.
I honestly have no idea what happens in other countries; I had never even thought about it, honestly. International events pay respect to both nations, to my knowledge.
I'm reminded of a quote from somewhere regarding flag burning, the content of which is something to the effect of "it's not about the burning of the flag, but the right you have to burn it".
nationalism at its finest, i was programmed as a kid to stand and remove any head gear for the anthem or i would be ostracized by others. its seen by some as basically spitting on the graves of the people who fought to make this country great. you mustn't disagree or that's unamerican.
I really don't understand the controversy over this happening to be honest. The national anthem plays every fucking game and both teams are from the same country. Fair enough if it was an international sport, but it isn't.