It's not fair to get pissed at anime and manga series that make inaccurate assumptions or say mean shit about America. How often do American TV shows, movies, and such say bad shit about other countries?
As for the comment about them being mad about Hiroshima, that's fucking low. Ever think that they could hate us for the same reasons the rest of the world hates us? No, it has to be because we bombed two of their cities. What-the-fuck-ever.
Besides, I doubt Japan is picking on America. They have a history of not being nice to foreigners. Well, Americans are foreigners.