Albamist,
If you are really interested in how the singing of the US National Anthem at sporting events got started, the answer pertains to WWI and baseball. According, to Ken Burns' PBS documentary on baseball, it was at a baseball game in Chicago, during WWI, that a band played The Star Spangled Banner during the seventh inning stretch, for a show of patriotism and support of the boys in France. According to the documentary, this had such an affect on the fans in attendance, that the Anthem began to be played at every baseball game. Now it's pretty much traditional in all sports.
BTW, the playing/singing of the Anthem is not required at any sporting event. In fact, during the Philadelphia Flyers Stanley Cup victories, their home games began with the playing/singing of God Bless America, by the late Kate Smith.
Robby