It's a "smoking causes cancer" situation. Just because other things also cause cancer doesn't mean that smoking doesn't. Read up on the history, look at each communist country, see what they become. Smoking causes cancer.
There's just still a bunch of communists who deny it because it's uncomfortable. Just like Neo-Nazis are denying the holocaust. If they were proud of it, they wouldn't deny it. They're not, but they'd like to keep the rest of their worldview intact and pretend that it didn't happen. Don't be like that, look at the history.