They did. People, white people included, have fought colonialism since it started. Some of the earliest documentation of protests against slavery and genocide in the Caribbean was written by Spanish settlers in the 1510s. When we talk about "white people" we are recognizing that whiteness is more about privilege than it is actual settlers who, while privileged by appearing white in the maintenance of whiteness as a naturally privileged category, can also have that privileged revoked if they become too much of a threat to hegemony. People make decisions based on their material interests, so a privileged class that is also racialized can be effectively mobilized to the disposal of the imperial state as their material security is assured or at the very least benefited by doing so. What we're seeing is people whose familial memory extends generations into that privilege realize for the first time that whiteness is in fact not a real thing that can protect you, and they think that's a particularly enlightened realization because many of them are also liberals who wish to appropriate ideas of anti-fascism and decolonialism to their benefit as they have effectively done in the past. They aren't mad about the violence, they're correctly identifying that they were promised protection from it by birthright.

Make no mistake, when they say that they were the first to realize the US empire was evil, they are asserting that they own the ability to criticize that empire and direct actions in response to that criticism. It's how they coopt black-, queer-, indigenous-, and disability-liberation movements all the time.