Maybe its different where you work, but we do not stand, pledge or sing at my place of business. We also don't sell tickets to the public to attend inside a city owned park.
When the team decides it wants to invite patriotic displays (whether its the Anthem, Kate Smith or generous donations from the DoD for flag unfurling and a color guard) into its event, the whole panoply of people's patriotic feelings and interpretation of duty comes into play.
Could they rule against such a display by their employee? Probably. I suspect it would withstand legal challenges and stand like the NBA's rule. But if they did, the patriotic display becomes quite hollow.