I went to a volleyball tournament in Manhattan beach and there was no national anthem played.
Also saw a bowling competition on ESPN no national anthem played.
Strange
Ive been asking this question on getbig for a long time and have not received a justified response. Shizzo said its because some sport players have been part of the military. I have worked with some people who have been part of the military and they don't stand for the anthem before they enter work. I will often hear people say, "Well, they are paying respect to the flag because freedom allows them to play football." But, freedom also allows me to go to work every day.
And, as you said, they didn't stand for the anthem before the volleyball tournament. So, why certain sports?
The real reason the NFL cares about the national anthem is for public relations matter. Good PR = more people watch = more money. That is what they care about. The NFL doesn't give a damn about our military--like most organizations, every move the NFL makes is to increase revenue. And, that is fine, but its rather tiresome when the NFL tries to pretend that they actually care about the military.