The Donald did not call anyone a "traiter." He expressed the legitimate and widely-held opinion that the National Anthem is a testament to the greatness of America, and is not to be disrespected, especially in this public forum (at NFL games), by people who are the manifest beneficiaries of the general color-blindness and generosity of the American people.
Indeed, in recent years, the playing of the National Anthem before professional sports games has been CLOSELY associated with American military heroism, to the extent that disrespecting the Anthem is tantamount to disrespecting those who have acted heroically for our Nation, often to the death.
And as with much of what Trump says, he was speaking extemporaneously, from his "gut," and not seriously suggesting possible breaches of contract by NFL owners (or the players, for that matter).
And BTW, President Trump [I love typing that phrase] has on a few recent occasions put to bed the fictitious "bro-mance" with President Putin through various military actions. Apparently you weren't paying attention.