The relationship of many American leftists to patriotism reminds me a lot of those guys who grow up in oppressively evangelical communities and decide to express their disillusionment with it by becoming black metal satanists - a oppositional mirror of Christianity - instead of becoming atheists.
Like the correct response to the realization that nationalism is a false consciousness used to suppress class conflict is to focus on raising class consciousness not to root against the nation everyone around you is rooting for.
Flag burning anti-patriotism is stupid in the exact same way that jingoistic chauvinism is stupid but it's also stupid in an additional self-defeating way because it sets your goals up in opposition to a bunch of symbols and institutions that normies have been trained their whole life to believe in.
It's especially self-sabotaging when the country is a democracy and you get some choice as to how it's run. It's no wonder voting for a better government seems asinine if you believe the country itself is fundamentally evil.