Not necessarily since one can argue that the National Anthem has nothing to do with their being hired to play football. Yes, they represent the team, the owners, and city, but was it a condition of their hiring that they needed to stand for the National Anthem?
Let's say that I get a job at the Metis Hospital.
You have a fine hospital, & it's popular with the residents of your diverse town.
I work there as the janitor, wearing your (hot pink) Metis Hospital uniform.
I'm a great janitor, always on time, always presentable, & doing the work of 2 normal workers.
On my way into work, I pause at the entrance, & address the arriving patients & visitors....
"Hi, I'm recruiting for the Nazi Party. We support the rights of straight white people."
This doesn't interfere with my janitorial duties.
It's before I start work.
Do you support my right to this political speech?