The propaganda model is based on democratic governments having no need to prevent anything. That the systematic bias is such that it's all going to be slanted toward state (see also: corporate) interests anyway.
Specifically about COINTELPRO, and someone may have more info on this than me, but I don't think it really became a story until people started winning court cases against the government about it. Just because information is out there doesn't mean it has to be reported (as Chomsky often cites to this day he's seen no reference to the US "invading" Vietnam in the mainstream press - we did invade, they just never refer to it as such - no one has to ask / force them not to).
.
A good of how it works is the Snowden leaks. Google / Facebook etc. would like very much for our intellectual property / personal information that they steal to be solely their property. Meaning not sharing it with the USG.
Google, I believe, has now become the largest corporate lobbyist to the USG. So since they're running the government (to an extent, anyway) and the mainstream media wants to please power, what is their reaction to the Snowden leaks?
They love them! Not because it's fighting power (NSA) but because it's what power (Google) wants. You take a look at Reddit's World News board - every single day for more than a year a Snowden related story has been featured on that propaganda mill of a sub-reddit. If it were actually something power didn't want known the leaks would have been buried.
.
Final example, one Chomsky gives a lot, is compare COINTELPRO to Watergate. COINTELPRO was a much more significant story yet Watergate was given 10,000 times more press coverage. Because half of the political power in the U.S. - the Democratic party - wanted that story to be big. Neither party wanted COINTELPRO to be big and that much larger story was only given a fraction of the coverage. Similar to Clinton and Monica - no one actually cared about that but half the political power in the U.S. wanted that story so it's all people heard about for years.
If neither party (or their corporate owners) wants a story it's simply not reported.