Do you not believe the US or the other imperialist nations have lied about their enemies at all? If not, do you understand why people should at the very least be suspicious of any claims made by known liars who have an interest in the narrative presented?
Not to say that any of these states have never done any wrong. None of us want the bad things and it's useful to critically analyze past failures to do better in the future. But it's hard to sift through the lies to find the truth so we can do that in any useful way.
Considering that the failures of the past aren't what we want for the future, demonizing these societies isn't particularly useful. It just serves the interest of the currently dominant powers. You can say that it doesn't and that you also don't like the US, but if your position on every country or movement that the US labels as an enemy is conveniently aligned with them... then... it's doing it's job whether you intend it to or not. It's just stoking more red scare sentiment, which gets used against even whatever you think "real leftists" are. It's not like the McCarthy hearings only went after secret soviet spies. It was just a tool to go after political opponents. It's this fear that allowed the US to justify becoming the biggest military in the world, invading tons of countries, spying on everyone, etc.
If we were to actually identify an agreed upon truth of some bad thing done by these states, I wouldn't disagree with you. However, in the context of a world dominated by the US and it's allies, this just doesn't serve a purpose. We need to work on ways to move forward and deal with capitalism. This doesn't do that. It just helps to justify the current world order.