It's actually not a free world. If you think otherwise you don't travel much.
It's a bad move to put your department at odds with peaceful dissenters and to do so publicly. It's a stunt, plain and simple. The entire "two sides" concept is the problem. There aren't supposed to be two sides. The cops aren't a business, they are public servants. They can buy as they choose, sure, but when they make the reason anything to do with politics, that's bad. Do you suppose all the people in that parish who pay taxes to support the police feel the same way as the sheriff feels?
He could have simply bought other vehicles and not said anything. He's only added legitimacy to the protest, same as Trump did. Eventually if you ignore it they'll stop doing it.
Cops aren't there to protect the flag, but to protect those they serve including those people kneeling.
That said, businesses are trying to capitalize on the fad. That's when you know it's a fad. It doesn't even have a message that's discernible any longer. It's now about sons of es and anthems.