2) If intelligence agencies are watching Twitter accounts for covert intelligence, that is idiotic. Twitter posts are public, easy to find, and unencrypted (I suppose you could hide a secret message in a Twitter post, but anyway...). It seems to me that the Rickrolling is perfect for disrupting ISIS sponsored Twitter recruitment accounts. When it comes to actually planning attacks, I imagine this makes no difference whatsoever--that is more likely done by ISIS on encrypted non-public channels that the intelligence agencies are trying to find and decrypt.
Except that they're not using encrypted channels to do the planning and execution. That's been made abundantly clear in the last week with multiple articles in the papers telling us as much. All of their chatter was done over phone texting. That's it. Nothing fancy. Nothing requiring any government to intrude on or break otherwise normal encrypted messaging. Maybe that's the problem. We've built up a boogieman in our minds that is this incredible supervillian-esque monster that's going to be doing everything on side channels with embedded encryption protocols and stenographic images.
I mean, that's what they're doing, right? It's what the old Soviet regimes were thought to do. Who knows, maybe these guys are just stuck in the world the way it was thirty or forty years ago.
To my mind it goes back to the OP's point. They're not using intelligence to stop these people. The question is it incompetence, malfeasance, apathy, or some combination of all three?