Isn't that a bit like suggesting that Christians love the Bible because of all the bloodshed and depravity contained in the Old Testament? Who even knows the words to the third stanza without looking them up? When I was in grade school, we sang the first and fourth stanzas only, and I doubt that one person in fifty could recite the words of any but the first. That passage in the third stanza may very well refer to the Black American slaves pressed into service by the British; Francis Scott Key was a slave owner, after all, and reportedly not a great admirer of Black folks in general, so it's doubtful he'd have had any qualms about sticking it to them a bit in his poem. Nevertheless, one obscure line from a verse of the song that no one ever sings proves nothing about anything.