It is my experience that people who feel the need to be extremely patriotic are also quite authoritarian. That’s not to say you can’t show your patriotism sometimes with a flag, but if you’re putting it on your truck permanently, where you’re neglecting the point that the flag is supposed to remain unsoiled and not be worn as clothing or a symbol, then you’re probably very much just jingoistic.
The 9/11 thing could be an exception, but I would also argue that it inspired a lot of authoritarianism. I mean, what is the Patriot Act but authoritarianism? So, sure, the base level of appropriate patriotism was higher, but there was still an authoritarian bent.
I mean, it makes sense, doesn’t it? A flat is the symbol of a country, and what specifically makes something a separate country? Its government. So while those who use it sparingly may be just celebrating some governing principles, those who use it a lot are celebrating the government–the authority.
And, well, Republicans and conservatives are actually more authoritarian, despite their claims for a smaller government. If they were truly libertarian, Trump would have been anathema. The idea of a strongman leader is anathema to those who primarily pursue personal liberty. The stronger the authority, the less free you are.
Now, granted, modern liberal thought has that at a balance, realizing that some government is necessary, and does not create predetermined lines like libertarians do. But it’s still all about freedom. The regulations that liberals want are ones that they believe allow more freedom.
I guess I’m rambling a bit, but I think you can see why it makes sense that the pro-authoritarian and pro-Trump style people would be the ones who feel the need to constantly signal their patriotism with the flag. It makes sense that these are the same people who go after others for being insufficiently patriotic. And that is very much a mark of authoritarianism.