Just like most Leftists do. Remember that the American Left typically hates their fellow Americans the way they accuse the Right of hating everyone else. It's a defining characteristics of the modern Left that they see hating their own demographics as a virtue. Non-American Leftists are more than happy to join in.
Of course Catholics have had a love/hate relation with America from the beginning. Some have tried to assimilate, but it's not been easy. From periods of open antagonism toward Catholics on one hand, to open antagonism from Catholics on the other, that's what's made being an American Catholic a tough sell.
Now in fairness, Mark's actual post praising Pope Francis as a Kairos moment in the flesh isn't so anti-American. But as usual, people use Mark's blog as a chance to spew all manner of hate, sin, wickedness, bigotry, and evil in his comboxes. Usually with impunity. Note the hatred, the contempt, the loathing. Note the 'Of course Americans are scum idiots, no wonder the world is justified to despise us and laugh at us.' Since hating stereotypes of people is apparently such a modern, progressive Catholic thing. I'll assume those Americans saying such things, like most demographic masochism, assume it's all the other Americans that deserve to be hated.