I'd also add that imbedded in the concept of "Make America Great Again," is a lack of pride in aspects of America today. So Trump is calling out Olympians for expressing things about his vision for America, while also critiquing aspects of the country.
From Langston Hughes' Let America Be America Again:
O, let America be America again—
The land that never has been yet—
And yet must be—the land where every man is free.
The land that’s mine—the poor man’s, Indian’s, Negro’s, ME—
Who made America,
Whose sweat and blood, whose faith and pain,
Whose hand at the foundry, whose plow in the rain,
Must bring back our mighty dream again....
O, yes,
I say it plain,
America never was America to me,
And yet I swear this oath—
America will be!