That's an interesting perspective. Still, in order for the anonymous source's testimony to carry any weight, there'd have to be some solid evidence. Either they'd have to show traffic logs, or they'd have to show the results of forensics done on the suspect's hdd. If you've properly distanced yourself from your activities, then there shouldn't be anything on your hard drive to implicate you.

Ultimately, it comes down to human fuckups. Bradley Manning is not in jail because he didn't take security precautions, he's in jail because he talked to someone he shouldn't have. The same will be true of any reasonably sophisticated hacking organization. They can take all the precautions in the world, but a vengeful ex can bring the whole thing crashing down.

If they were serious about the investigation they'd probably get a sneak and peak warrant to install a keylogger/etc. (either electronically or physically) instead of just a smash and grab. That way they'd have some good trial evidence, because as you say relying on forensics for computer crime is quite dicey - something the bureau is all too familiar with. If they can beat the system electronically this also gives them a chance to see whether there is anything that'd make a quick seizure worth it.

The law enforcement exposure they just did undoubtedly made it much easier to get warrants or FISA approval if they did have some targets.