Not being a USAian, I find this deeply bizarre. As far as I'm concerned, when one's national anthem is played, one stands up, shuts up, and refrains from fidgeting. (If one knows the words, one may sing along.) Rules about how one stands up are an alien concept.

(This is why I was quite annoyed a few months back when the breakfast DJ decided, for some unfathomable reason, to play the Irish national anthem while I was getting dressed. Standing up and staying still when semi-clad in a chilly bedroom is annoying.)