If they were around when the wars happened, perhaps they'd have a fair excuse. I know my Granddad to his final day wouldn't stand Germans, but he fought against them, they sure would've killed some of his friends. Perhaps its still not right, but he would have a reason.
I suspect the more likely case is however that these people are using patriotism as a mask for racism and bigotry, as those people about the Navajo Wars, The Banana Wars, in WW1 the Baralong Incident and the use of chemical weapons in the war and the notable amount of US war crimes in WW2 such as in the Moro Rebellion, the incident with the USS Wahoo, the US rapes of Okinawan civilians, the rapes in Europe by US soldiers, including of allied individuals! The dehumanization of German POW, other massacres of civilians, several of which were in Asia.
And obviously, how could we forget the biggest two failures of the US when it comes to foreign policy. The numerous, and often unjustified invasions and interventions of the Middle East, which essentially gave birth to the terrorism we suffer in the modern day and finally, the uncountable injustices by the US in the Vietnam war.
Those sort of people just choose to acknowledge the injustices committed against the US so they can discriminate against foreigners or people who don't fit into the model of a stereotypical US citizen, these are the sort of people who get leaders like George W. Bush, Nixon (to an extent) and Donald Trump...