If we ignore that corporations have some ability to control employee behavior during office hours, and just assume that the NFL players have a right to political statements even during their paid hours, then Trump's message to his base that they have political power by not buying tickets to an NFL game, or otherwise paying for a game in ESPN is also a valid political statement.

It is basically a potus asking his supporters to boycott NFL games until they make a decision on whether to allow politics during paid hours.   Is it divisive?  Sure it is.  

It is valid to discuss  if the flag  is or is not sacred and whether people can exercise political and economic power by withholding their purchasing.   Some people are just angry that Trump is pointing out that his groups of supporters also can organize the same boycotts of NFL teams.  

The actual impact in coastal towns where very few voted for trump is at best 10% lower revenue.  In Trump states, it could be 50% of revenue.