I was arguing with someone about a fringe website the other day, and I think I finally understand the thought process a lot of these people have. I think many are people who are strongly and habitually opposed to something (in this case, US injustices) have that opposition warp their perception of other things (especially things "opposed" to the US). Sometimes that means they see discussion of other injustices carried out by US adversaries as a distraction to be fought. Other times, that means denying and defending against accusations of injustice against a US opponent. In extreme cases, it means starting to explicitly support those opponents, even through they're arguably worse on almost every value axis these people started with.
tl;dr: anti-imperialists who oppose the US imperialism can sometimes become defenders of imperialism done by US adversaries.
This is article that helped me realize this:
https://newpol.org/against-the-grayzone-slanders/:
> During the Vietnam War, opposition to the United States and support for national self-determination for the Vietnamese among young activists sometimes developed into support for the Vietnamese Communist Party and its leader. One often heard at the protest demonstrations, “Ho, Ho, Ho Chi Minh, the NLF is Going to Win.” While this drift from opposition to the U.S. government to support for the Vietnamese government was understandable as a gut reaction to the horrors of the U.S. war, it was also very problematic. Most people chanted this slogan naively, but some gradually became supporters of Ho Chi Minh, the Vietnamese Communist government, and of the Soviet Union that was backing it. Few in the movement pointed out that Ho and the Vietnamese Communists were Stalinists who had murdered their leftist competitors and strove to construct a Soviet-style state in Vietnam, which is eventually what happened.