Freedom of expression does not mean that a person may not have to face public condemnation. If you express unpopular views, or just views detested by a large number of people (who may not be the majority), you can expect to be condemned publicly. Liberals and progressives are just fine with this principle for views they detest -- Republicans, Bible thumpers, white racists, Donald Trump. In many cases they demand not just condemnation but the suppression of such views by the State. It's only when opinions they agree with encounter pushback that they become more purist, though they are ready to demand the suppression of the views of their critics, as King did.
The First Amendment, and the general principle of freedom of expression, assume that there will be public debate, without making any assumptions about the quality of that debate. (And a good thing, too, since the level of public debate is generally not high.) What is important is that someone should be able to express a highly unpopular opinion without being silenced -- by the State or any other force. Someone who wishes to express a highly unpopular opinion had better expect to encounter hostile responses; one very annoying tendency visible among liberals is that, for example, they should not be made to "feel like an outcast" (via) for taking an unpopular stand. This would be bad even if they didn't feel that no such consideration need be extended to those whose opinions they hate.
It's to their credit that the athletes themselves, as far as I've seen, don't seem to be demanding that they not be criticized. Perhaps because most of them are black and are therefore closer to political struggles of the recent past, they knew from the outset that standing up against the majority would make them lightning rods for hostility.
Now I'll address what I take to be King's more specific reference to President Trump's attacks on the athletes who protest against American white supremacy, while generally supporting American military aggression. It's true, as the friend who posted the tweet on Facebook argued, that the words of a President carry more weight in the public sphere than those of most citizens, though not (as she also argued) that they take "the form of law." Admittedly, partisan fans of a president will want to see them that way. But my friend, like so many Democratic loyalists, wasn't nearly as concerned about (for example) President Obama's prejudicial remarks about Chelsea Manning or Edward Snowden, let alone Obama's general war on whistleblowers. Privately, she probably would have agreed with them even if they had the imprimatur of a president she admired, but like most Democrats she ignored them or minimized their impact on the well-being of people who'd been accused of crimes. Nor, if I recall correctly, did she object to Obama's public criticism of Fox News, though right-wing partisans reacted to it in much the same panicky way that Democrats are now reacting to Trump.
Though I agree that a President's public statements will carry a lot of weight, it struck me funny to see my friend making the claims she did just as the owners of the NFL, and the NFL commissioner, struck back at Trump's demand that protesting athletes be fired. Former NFL coach Rex Ryan, who'd campaigned for Trump, announced that he was "[bleep] off." (Presumably bleeped by ESPN, where he appeared, rather than by Democracy Now!, who quoted him.) Pushing back against the Leader of the Free World is harder than pushing back against a single football player, but it can be done, and it's being done. (I'm with "former NFL player Donté Stallworth," who also appeared on DN! this morning, and warned against letting Trump hijack the protests into a controversy over himself, though that already seems to be happening.)
P.S. When I pointed out some of this, my friend replied that I should "tell it to the Joint Chiefs of Staff" in connection with Trump's announcement via Twitter last summer that transgender troops would no longer be allowed to serve in the military in any capacity. This was a notably ill-chosen rebuttal, because, first, Trump sent that tweet as a declaration of policy, which was not the case with his denunciation of the NFL protestors; and second, because the Joint Chiefs did not accept the tweet as having the "form of law." They announced that until a policy had been worked out formally, they were going to ignore Trump's announcement and transgender troops would continue to serve. Until Trump signed a memo implementing the ban, it wasn't law. The tweet itself did nothing. This case also supports my general distaste for the hopelessly inadequate way liberals have been responding to Trump's provocations.
