Even assuming there would have been a valid reason for law enforcement to disrupt the communications of those individuals, how could an intelligence agency be justified in doing so?
* Though through inter-agency collaboration, they tip off other agencies when they have something...
With that out of the way, they're targeting the former in-order to make it impossible for people to come together and organise privately and anonymously around subversive ideas.
The specifics of targeting IRC are probably tied to the efficiency of the protocol which allows very cheap hardware and minimal bandwidth to the extent that non-complying private foreigners may provide a free forum the government can't control.