You know, I joined the military in the early 80's, served 20 years, and retired in the early 2000's. Over that 20 years I was in, I served in 4 different combat zones.
Every year, there was a half a week of training that we did called Navy Rights and Responsibilities. That was training on what was expected of us, how we were to conduct ourselves, as well as a lot of other stuff. Some of which was about the Constitution and our Oath of Enlistment. When I enlisted, I swore an oath to support and defend the Constitution of the USA, against all enemies, foreign and domestic. My oath was to the Constitution of the US, not the flag, not the anthem, but the founding document of this country.
Part of that oath means I support and defend EVERYTHING in that document, which means if someone wants to do a peaceful protest, they have the right to do so, whether or not I agree with it. So, it's because of the service of people like myself that allows her do take a knee if she wants, because the Constitution (the document I swore an oath to support and defend) says she can.