[D4M14N]

The little known law they quote is actually very well known if you are an ISP / carrier. But the law is not a legal mandatory filter requirement - just a requirement that you "reasonably provide assistance when asked" It just happened that most of the ISP's when asked in this case decided to comply, but importantly not all complied.

[caf]

Well, what exactly legal obligations does that requirement impose on the ISP? If they're allowed to refuse any request, then the law wouldn't be needed in the first place - government agencies can certainly send whatever non-enforceable requests they like without needing a specific law to let them do so.