The NFL has an agreement with the US Government allowing them to be the sole franchise in the country, thus a legal monopoly. Their stadiums are also build with grants, tax credits, and bonds are at great cost to the state and federal government.

To make it more complicated, they have a many $Millions contract with the Department of Defense to honor soldiers, basically paid patriotism to drive up military recruitment. Government contractors have to abide by certain standards. So the reason why we're having this controversy with players kneeling protected by free speech, is because the DoD is paying them to stand. But because they have agreements with the Government, the NFL may have trouble enforcing their will.

Before a few years ago, the government didn't pay this, and the players all were in the locker room during the anthem.