But that is up to their employer, not POTUS.
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Trump, as usual, has opened mouth and inserted foot without an even a scintilla of intelligence. If he truly believed that it is important to stand during the anthem then he once again has done far more harm than good.  This form of protest was already on its way out. In week one there were only 5 or 6 players in the entire league that knelt or sat during the anthem.  What's more, if the owners wanted to make policy to have their players stand or be fired, they could have.  But now that Trump has inserted himself he has made it much harder politically and legally for the owners to do that.
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For all those that are rushing to agree with him, you are being duped. Trump is really only doing this to foment his base of intellectually challenged followers, that like their leader are easily distracted with emotional irrelevant nonsensical rhetoric in order to keep them from noticing the mounting failures and bankrupt ideology.