@edgarblythe,
There is nowhere it is written that I know of, but it is more powerful trend than just a traditional custom.
In grammar school in the ‘50s, our teachers ‘made us’ recite The Pledge of Allegiance. Was it required? Effectively yes due to social pressure. As well, so was the recitation of the 23rd Psalm (Our Father). As a Jew, I wasn’t required to say it, just bow my head and have hands folded, but that is another kettle of fish.
Furthermore, as is performed at sporting events, the Pledge of Allegiance wasn’t required but the singing of the Star Spangled Banner occurred every single game. In NHL Hockey games, it is both anthems that are sung. Clearly, it’s a statement of national support and allegiance.
Should we do this? Not sure. There is no law, though.