you're allowed to be defensive
You say this, and then you criticize them for being defensive.
If someone walks up to you and says you're a piece of shit, responding with "You're a piece of shit! It's inappropriate to call people pieces of shit!" Does not actually improve the situation and is a little bizarre.
Then you follow-up with criticizing them for being defensive after you called them a piece shit in return and it's just weird.
And they didn't walk up to you. You went into the "Fuck the USA" community and are clutching your pearls that you heard someone say "Fuck the USA." What were you expecting to find here?