From what i've observed the answer falls into a couple of camps, 1. ML's usually tend to back Russia and Iran due to their opposition to western hegemony even if they accept that they aren't progressive due to a general understanding that the no.1 enemy of socialism in the modern era is american imperialism and thus their fight against it is beneficial in the long term, even if they preffer that they were socialists
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The second camp is the nationalist communist types, for example the ACP, they generally tend to view russia and iran positively precisely because they are socially regressive
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The general idea of “the truth is on the side of the proletariat, not on the side of any government or capitalist.” while accepted as correct, it's viewed negatively due to a tendecy of certain western marxist schools to call for revolutions in already socialist states like Cuba or China and their tendency to infiltrate and sabotage parties that support the existing socialist countries and anti-imperialist struggles against the US and its lapdogs.
It also doesn't help that these schools of thought were actively financed and guided by the CIA to undermine the various socialist movement in the west, so the support of anti-imperialism even if done by coincidence like in the case of russia and iran became a way to see if someone is actually comitted as a serious socialist and not a government spook