Because Communism is a death cult that seeks world domination and mass murder of their "class enemies." What is a ruse has been Perestroika and capitalization in these countries. Lenin himself, in his New Economic Policy, reversed communism temporarily in order to industralize Russia, using disinformation in order to attract western money and technology to do so.
These are totalitarian regimes which put out the sort of anti-American propaganda that supports genocide against our people. The day they feel they can kill us off without suffering a devastating retaliatory strike, or can do it while denying responsibility, they will do it.
Right now, China is pretty much Communist in name only. The only reason China has a strong economy is because Nixon stupidly opened them up for trade with Capitalist Westerners. Otherwise, Mao’s policies were taking them down the crapper. With their “economic opportunity zones” China has opened the Pandora’s box of Capitalism, and the Chinese people want a slice of the good life. If we foment Chinese dissent of our own, then we keep Beijing occupied with retaining power rather than world conquest.