Couple things:
While many child porn rings are very careful about their access and sharing, there are many people out there whom we can classify as "The dumb ones." These are the people the FBI is catching... the ones who aren't terribly smart about it, haven't been doing it long, and aren't the actual creators or perpetrators. That said, they are still downloading and viewing child porn which is illegal and abhorrent. So, I have no sympathy.
Second, while Slashdot and other news sources are great for posting brief summaries of law enforcement initiatives, they rarely (if ever) get a complete picture. In my experience, there is usually a lot more to enforcement and intelligence activity than what gets reported in the media. I would venture a guess that this is such a situation. Most online "stings" conducted by the FBI and similar authorities have been reasonably complex and well-targeted affairs. While checks and balances are undoubtably necessary, the number of false positives their operation will generate is likely quite low - this isn't going to be a matter of posting fake links in the wide public spaces of the internet; they will target online communities and known pornography sites with material that hovers on the fringe of legality in the first place.
It's a fairly safe bet that when they go through the effort of pulling ISP information and a search warrant, it's not going to be done lightly.