Based on erotic manga by Yabitsu Hiro.

Why is there so much anti-Americanism in anime?

Well there's a lot of anime and manga out there with a variety of depictions of Americans, some even starring American protagonists.
But to cut things short and answer the question lets look at a particular anime that has a pretty negative portrayal of Americans, Ghost in the Shell: Standalone Complex The very first episode starts off with the show's Japanese police protagonists investigating a gruesome murder scene, a Japanese woman with all of her torso skin missing.
It turns out a mentally deranged American ex-Soldier immigrated to Japan and started murdering women by peeling their skin off while they're alive.
The method he uses though is suspiciously similar to war crimes committed in the American Empire's invasion of Mexico a few years back.
One of the Japanese police, Batou, was a UN peacekeeper sent to protect civilians and feels personally incised to bring this killer to justice. Batou has flashbacks to a village captured by American troops The American Empire's CIA agents show up to 'assist' the Japanese police and insist that once this American serial killer is captured he will be extradited to the US.
This angers the Japanese police who feel the Americans are disrespecting their authority in their home country. It turns out the American was part of a terror squad sent into Central America to butcher women and children and break the morale of the populace to force a surrender to the American Empire.
, and the CIA knows it.
They were counting on Batou to go into a frenzy and kill the American so he won't leak any information about American war crimes during the Mexican invasion.
The CIA's plans are foiled when Batou remembers he is a cop now, not a soldier, and the American is captured and retained in Japan for questioning. The American puts on Kabuki makeup before beginning his ritualized killing routine Something else about the episode stands out though, the CIA agents sent to Japan had a very 'curious' look. The CIA agents give their names as "Suzuki Sato" and "Tanaka Watanabe".
Those are actually common Japanese surnames mashed together and sound fake, the American equivalent would be something like "Smith Johnson". They then do little bows and there's a close up of Tanaka's face as he repeats "Pleased to meet you, so pleased!" Every Japanese person they meet feels really uncomfortable around them.
Why? These American Empire agents are a walking stereotype of Japanese as portrayed in American media: Note the coke-rim glasses So you get the impression that the Standalone Complex writers were familiar with how Americans stereotype their people, and this was their jab back at them. A backstory element of the Ghost in the Shell setting is that America's economy is in decline while China's is now the world's largest, so Japan is warming up to China while America gets increasingly desperate and hostile to make themselves "great again", like invading Mexico. Portraying Americans as the "other", the "enemy" Saito is an expert marksman with the Japanese police.
In the episode "Poker Face" tells the story of how in the past he became a mercenary to help Mexicans fight off American invaders.
Saito became even more determined when he realized that the Americans, who have roped their usual lackeys Britain and Japan in as "UN Peacekeepers" were planning on using nukes. "Those filthy bastards had the balls to bring in a tactical nuke.
That gave me determination, I was planning to put the fear of god into them on high" -Saito recalls the disgust he felt towards the Americans as he prepared to kill them "They're always worried about catching flack for attacking churches, hospitals, and whatever, they left too many places for guerillas to hole up in" -An American soldier comments on how they're not being hard enough on the Mexicans A minute after uttering that line that American gets blasted through the abdomen by Saito, leaving him on the ground wimpering "WHY...
ME..." The sergeant of the group, White American then commands the only two Black members of the group to go run into the open and rescue the downed White American.
One of the Black soldiers says under his breath "Bigoted son of a bitch..." but obeys.
Afterwards the White sergeant refers at the Japanese members of the UN peace keeping group as "Hey, you japs!" while barking out another order. Eventually only the Japanese members of the UN team are left alive, including the Ghost in the Shell protagonist Motoko Kusanagi.
Turns out this is a "how they met" story. Kusanagi out guns Saito and instead of killing him, recruits him to work for her.
She wasn't there to fight for American interests and had zero sympathy for her dead 'comrades', she was there to recruit good soldiers for a special Japanese covert operations group.
And possibly make sure the nuke doesn't get used/misused by Americans. This seems like a extremely negative portrayal, even hateful portrayal of Americans (Whites in particular), but it does feel kind of familiar... It was Full Metal Jacket from the Asian sniper's perspective! This time it was the Americans who were the "other" being gunned down as the Asian protagonist narrates his thoughts.
The protagonist then gets to survive and reflect on his experiences while his enemies who only existed to be shot at for this scene remain corpses. Now think back to the American entertainment you've seen, all of the times faceless non-Americans are blown away by protagonists with no sympathy.
For American audiences used to always being the protagonist, the hero, or the victim to feel sympathy for, being the faceless 'other' can be a jarring experience.