First of all, they are not protesting the Star Spangled Banner, they are protesting perceived difference in treatment by police offers of blacks versus whites. The people who are upset at these protests are upset at the forum they are using.
And while yes, the anthem was written to inspire patriotism, nationalism, and American Identity. And yes, the song includes lyrics dedicated to slavery (fairly common at the time), the song isn't a "slave propaganda song". It's a nationalist propaganda song, just like God Save the Queen and O Canada.