He's right. I'm one of those veterans who made a career out of the military and I am not offended by anyone taking a knee.
Why? I swore an oath 5 different times, to support and defend the Constitution of the United States of America. That means I support and defend what is contained in that document, and 2 of those things are free speech and the right to peaceful protest. The fact that people feel secure enough in their rights to use them means that my service wasn't in vain, and the Constitution is still the law of the land.
As a friend of mine once said "I may not agree with what you have to say, but I will defend to the death your right to say it."