Donald Trump was defending the honor of the American flag when he called for NFL owners to fire any "son of a bitch" who doesn't stand during the national anthem ... this according to the White House press secretary. 

Sarah Huckabee Sanders just fired back at reporters during a White House press briefing -- where she was asked if Trump's 'S.O.B.' comments crossed the line. 

"It's always appropriate for the president of this country to promote our flag," Sanders said. 

"This isn't about the president being against anyone. This is about the president and Americans being for something."