[TheRealVille]

After looking at the 36 usc 301, I don't think it's a law, but rather a code that we should honor. "Should", not "must" is key here. Still, I can't understand why someone wouldn't want to do it.

Sec. 301. National anthem

-STATUTE-
(a) Designation. - The composition consisting of the words and
music known as the Star-Spangled Banner is the national anthem.
(b) Conduct During Playing. - During a rendition of the national
anthem -
(1) when the flag is displayed -
(A) individuals in uniform should give the military salute at
the first note of the anthem and maintain that position until
the last note;
(B) members of the Armed Forces and veterans who are present
but not in uniform may render the military salute in the manner
provided for individuals in uniform; and
© all other persons present should face the flag and stand
at attention with their right hand over the heart, and men not
in uniform, if applicable, should remove their headdress with
their right hand and hold it at the left shoulder, the hand
being over the heart; and

(2) when the flag is not displayed, all present should face
toward the music and act in the same manner they would if the
flag were displayed.