And even if you did run across it, and quickly backed out, then suddenly got searched by the police and had a couple of images in your cache, the police are quickly going to realise that you're not the person they're looking for.
Hundreds of images in folders, different story.
Looks like you've just pointed out another flaw: it's introducing a quick and easy way to troll people outside the boundaries of the internet without having to leave the keyboard...
The loquacious version for the less technically inclined: your computer is not secure. Not even slightly. The ease with which malicious software can be installed on your computer cannot be overrated. Said malicious software that has the potential for someone to completely control your machine without your knowledge. To make as many requests and save as many pictures from as many websites in as many folders as whoever is controlling it may feel like.
While it is arguable that people can do this now, there is currently no real valid way to anonymously tip off the police. An investigation will not be conducted from a random call from a phone box. Details of how they have knowledge of their alleged activities, etc. will at least be sought before anyone even lifts a pen to write down a name.
With instant flagging / monitoring of people from their browsing, this has removed the greatest barrier to entry - aside from the technical knowledge, but plenty of people have that - of someone who has a grudge against you totally ruining your life in a matter of minutes.
Hell, the person doesn't even need to have a grudge against you. The internet has nurtured a particular culture of schadenfreude known as "lulz" wherein the aim is to upset and grief as many people as possible by whatever means necessary. This is well and truly giving people means - government sponsored ones.
Hmmm. Somehow my explanation became a novel. Sorry about that, but it's important people are aware of all the potential forms of abuse that can arise from this.
I'll just say to the "It's worth the risk in the name of the children!" replies, please believe me that this piece of technology will in no way hamper people who actually desire to see illegal sexual material. Even if child pornography rings did just consist of a website that you went to (they don't), bypassing the filtering is so simple that any teenager who has ever wanted to access Facebook at school during class hours knows how to do it. Why bother?