Those slaves and servants were recruited as soldiers in the British army. The british, you know some of the people that originally profited off the slave and indentured servant trade, weren't doing it to be nice to them. They turned them into combatants. There were slaves and servants who fought on the half of the british, and there were those who fought on the behalf of the Continental army.
By the way, you seem outraged about the fact he's referring to "hirelings and slaves" and assuming he's only talking about black people, but I'm curious to know if you're equally outraged about the fact that thousands of white indentured servants, who were essentially slaves themselves, were also offered freedom if they fought for the British. Who do think the "hirelings" were?
Yes, It is your anthem and mine as well, but technically, it's from the early draft of the poem. Just like we added to amendments to improve our country, the song was amended to reflect contemporary America. And the song wasn't even our anthem until about 80 years ago, only the branches of the military sang it predominantly.
We're Americans. So we can either bytch and complain about what people did wrong several hundred years ago and call each other "motherfukkers" on an internet message board, or we can admire the things our country did right and amend the mistakes of the past.