A flag is a symbol. Words aren't just a bunch of letters, there's social meaning in the unique message conveyed.
But yes, that nitpick aside, it's absurd nationalism at play. When someone burn the flags and books of my political alignment, I couldn't care less. Sure, it's an offensive gesture to burn a symbol, it's not crazy for someone to get mad at a burned bible or national flag. But at the end of the say, I see it as no different to someone saying 'America sucks', which is pretty common.
And the weird nationalism is pretty internalized to the point where it's just normal to most Americans. They point to the nationalist displays of other countries like 'Cult of Personalities' elsewhere and don't notice their local fixation with Washington and the rest, or reciting a daily oath to their flag, or flying them everywhere constantly all the time. But from an outsider perspective, those rituals are just... concerning. It's ingrained nationalist propaganda.
If you worship any flag or take offense to any flag, you need to get a life.
This I disagree with. A flag is a symbol, it represents concepts. And some flags can represent, among other things, "I want you killed" (consider the Nazi swastika being flown today). I think it's reasonable to be offended by a flag representing an offensive idea.
But a national flag flown by the world superpower? It's a bit fragile to flip out over that, isn't it?