They don't have to be fired; if they don't like the companies policy they can always quit.  After all the name of the origination is "National".  They keep score, have a playoff system, rules etc so it's not like there are no company policies or regulations.  
As an owner, you have for over the life of the organization, have had an unwritten or public implied rule to stand and acknowledge the flag of this great nation. If social demands require that you now codify this acknowledgement, then so be it.  Get er done, that flag is a lot more valuable than your franchise to this country.