Ignorance on Parade!
I know they teach neither reading comprehension nor history in the public schools anymore, but this isn’t that hard... did they read the entire stanza, or just the line with the trigger word “slave” in it?
The verse is speaking about how there is no refuge for those who would seek prevent our new country from rising... how their blood would flow in the vane attempt.
The British employed hired mercenaries and slaves as fighters... thus “hirelings and slaves.”
The freakin verse is talking about the British... not us... and certainly not the slaves in America at the time!!
Come on people....