There is no mandate, there can be no flags...
It's no longer about respect for the Office of the Presidency, it's about beliefs. The New England Patriots did not become Super Bowl 51 Champions in a stunning come-from-behind win without believing in themselves. So why would a visit to Donald Trump's White House be any different? Successful football players have strong beliefs and if they are at odds with the current President and his policies, they can choose not to go.
Tom Brady chose not to visit the White House when Barack Obama occupied it in 2015. Brady exercised his right to choose and you do not have to agree with him to respect his decision.
Roger Goodell's NFL mandates many things, like interviews on Media Day before the Super Bowl. He has not mandated the trip to the White House by the Super Bowl Champs because that's a political choice. On record, the trip to the White House is by invitation, and invitations can either be accepted or declined. Several New England Patriots players have exercised their right to choose and have declined President Trump's invitation.
In a lot of ways, these players in declining the invite, are doing so out of respect for the many citizens of the United States they believe, have been disrespected by President Trump, his policies and his cabinet members. They do not agree with the politics of Donald Trump's White House.
History has a funny way of remembering events and in a lot of cases, it disagrees with the headlines of the day. The players who have declined may be under scrutiny today for their choice, but years from now, after all the truths are revealed, they may not be remembered simply as Super Bowl 51 Champions, but as courageous champions of the oppressed.
I would not call the Patriots players who declined a trip to the White House, including Brady in 2015, radicals, but I would rather refer to them as activists. They're making a statement that not every Super Bowl Champion has to "go along for the political ride".
You don't have to agree with their politics to respect them for holding onto their beliefs and making the right choice for them. There is no need to throw the flag...