The third stanza of the US National Anthem, The Star Spangled Banner, read as follows:

And where is that band who so vauntingly swore,
That the havoc of war and the battle’s confusion
A home and a Country should leave us no more?
Their blood has wash’d out their foul footstep’s pollution.
No refuge could save the hireling and slave
From the terror of flight or the gloom of the grave,
And the star-spangled banner in triumph doth wave
O’er the land of the free and the home of the brave.

Of concern is the clause:

No refuge could save the hireling and slave
From the terror of flight or the gloom of the grave

It is assumed that this means, because it mentions slaves and hired mercenaries serving the British, that slavery is here justified and thus demeaning to the people.

Of course nobody discounts the facts of history. Francis Scott Key was a slave owner.  But was his statement here a generally derogatory statement or something else?, though generally liberal in its outlook, got this one pretty close to right:

It goes without saying that Key did not have the enslaved black population of America in mind when he penned the words “land of the free.” It would be logical to assume, as well, that he might have harbored a special resentment toward African Americans who fought against the United States on behalf of the King.

The post also goes on to say that

In fairness, it has also been argued that Key may have intended the phrase as a reference to the British Navy’s practice of impressment (kidnapping sailors and forcing them to fight in defense of the crown), or as a semi-metaphorical slap at the British invading force as a whole (which included a large number of mercenaries), though the latter line of thinking suggests an even stronger alternative theory — namely, that the word ‘hirelings’ refers literally to mercenaries, and “slaves” refers literally to slaves. It doesn’t appear that Francis Scott Key ever specified what he did mean by the phrase, nor does its context point to a single, definitive interpretation.

The language of the verse gives reference to the historical fact that the groups (whether two or one) were under British employ. All else is speculation.There is at a minimum nothing here intended to further demean the slave in the U.S.  Of course the problem of slavery itself is not dealt with in the song and one might even say that unless he challenges slavery that he is supporting it, which was his lifestyle.  Whether or not that is the case seems beside the point. Key’s personal errors on slavery do not alter the problem of the British conscription of slaves. That, again, is merely a fact of history which is recorded in the verse.

Key was, then, clearly referring to those hired or conscripted to serve the British. Nothing else is reflected in his lyric. It would be another half century until the institutional situation was resolved.

What Kaepernick missed was the Republican party of old, the historic conservative movement.  There’s Lincoln, of course.  There’s President Grant shutting down the KKK (until it was revived through the Democrat & liberal progressive Wilson). And Frederick Douglass marching with James Garfield because of his efforts. Even before the Civil War there was the evangelical Methodist abolitionist movement in Kansas that fueled the whole war thing. Then later people like Mildred Jefferson.  Theirs might best described as “historic Christian progressive” ideals. Like Wilberforce and Newton (Amazing Grace). It’s a rich history. This the modern conservative movement (not the Republican party) with its evangelical influence has more to offer.  Oppression stops here.