The issue isn’t the NFL, NBA, MLB or the fans standing next to you. The issue is the US women’s soccer team.
Do they normally all sing? If so, then half of them not singing may very well be a protest.
No, they don’t normally all sing. They are modern American young people and they behave like their peers, both the athletes on the field and the fans in the stands.
Americans mostly stand for the national anthem. Very few of us actually sing it, and we haven’t for a long time. Why would anyone expect a women’s soccer team to be any different?