If you read the article, you have to note that the "DA" that you abhor hasn't been involved at all. This was a crime unit who uncovered a ring of pedophiles, but who can't do anything about it because our laws require catching the pervert in the act.
Well, it does and it doesn't. I think as much as anything else, the article sort of oversimplifies the situation here. The mere act of subscribing to such a website is not, in and of itself, illegal. What is illegal is possessing child pornography. I think what was meant by "catching them in the act" is that if you have someone that you know is currently browsing the site, then you know they are currently in possession of child porn, thanks to their browser cache, if nothing else. Thus, you could coordinate an arrest with their browsing activity.
However, what I don't understand, is that finding someone's name and credit card number in the records of a site that deals in child porn should easily be grounds for a search warrant. So why not get warrants and start searching computers? If they have something there, you arrest them and prosecute them, and if there's nothing, you let them go and hope they slip up sometime in the future. I can't for the life of me understand why they wouldn't do that, unless the sheer numbers of people involved would have made it impossible - perhaps they were worried that they would nab a few folks, and then the word would get out that the police were on the move. And then, after that, they'd find nothing but a bunch of cleanly-wiped hard drives, and end up wasting a lot of time. But it still makes sense to me to try....