Well, it’s mainly a personal (and, i concede, largely ineffectual) protest against the sort of excessive nationalism that anthems tend to reinforce, particularly in countries like the US and Australia, where i’m originally from.
The anger and hostility displayed by some people towards those who don’t stand is, it seems to me, pretty good evidence of the excessive deference to symbols of authority shown by certain sections of the population.
People say that standing is a sign of respect. Well, put simply, i don’t stand because i don’t have a great deal of respect for the relatively abstract concept that anthems tends to represent. I don’t respect “the United States” or “Australia,” because to me such a concept tends to imply unconditional support. I respect certain people and organizations and ideals and actions within those countries, and i dislike intensely other people and organizations and ideals and actions. I feel no need to participate in a ritual that, to my mind at least, reinforces a veneer of unity while glossing over important and problematic divisions.
I’m quite aware that my personal attitude to anthems is just that - a personal one. I don’t expect other people to see things the same way i do. Plenty of people stand up for plenty of good reasons. I respect their decision to do so, and when anthems are playing i stay seated and quiet so they can pay their respects.
The closest analogy i can think of is a religious one. I’m an atheist, and if i happen to be visiting a Christian household where a prayer is said before meals i will sit quietly and wait for the prayer to finish. I will not, however, bow my head, close my eyes, pretend i am praying, or say “Amen,” because i prefer to be honest about my beliefs. I will just stay quiet and allow everyone else to pray or not, as they see fit, the same way i sit at the ballpark while others choose to stand.
And finally, off the topic a bit, i reiterate my earlier point that, the way the Star-Spangled Banner is sung at most sporting events, it has lost much of the dignity and the public participation that was meant to accompany an anthem in the first place.