All that Parenti is highlighting, is that its very easy to close down discussion of certain topics in the US, by bringing up the word "conspiracy". Indeed with many people (reasonable "liberals" mostly), their internal censor does it for them. That's a very different thing, and is the fault of the conspiracy theories that you are trying to defend. Its because of wackoes, that conspiracy has become a dirty word.
Note, none of the events described by Parenti happened in secret. It all involved the organisations you would suspect, and the discussions took place in public. The more interesting question (and one that Chomsky, amongst others, have tried to answer) is how they can take place so openly, and yet be ignored. The other question, is why conspiracists never notice these things, either. Perhaps open conspiracies aren't sexy enough - they'd rather be battling secret subversives for the soul of the world. Who knows, certainly the psychology of the crazed is not my area of expertise.