By taking the extra step of calling for players to be fired, he stoked even hotter outrage from all the people he intends to inflame, a strategy of his that works time and again. He wants his most liberal enemies in politics and media to take the bait on these cultural war asides, and they usually (and stupidly) oblige.
These battles illustrate why the Democratic Party has "lost favor among voters in rural and small-town America," as legendary journalist Dan Balz reported last year. People don't take kindly to not standing for the National Anthem in all the places Trump and the Republicans need to win in 2018 and 2020. Trump couldn't care less if people in urban areas, where Republicans have no hope of competing anyway, are upset with his strident, colorfully communicated views.
Like a political magician with an endless bag of predictable-yet-entertaining tricks, Trump turned Democrats into the urban elite, anti-working man party in 2016.
In 2017, he has turned them into the anti-American flag party.
If Democrats don't stop taking the bait on these things, for his next trick, Trump might just make them disappear in 2018 and 2020.