Police and intelligence agencies obviously do have the resources needed to remove illegal websites from the Internet. Yet many of them remain, often for a long time. It is possible that the police lets them remain as a strategy to make people who use the Internet accidentally stumble into them. This makes lots of people "criminals", the "benefits" of this to THEY is explained in another article, sick law conspiracy.
Classified information is one example of this. There are many examples of classified information. While those with power would of course do their best to remove important secret information from the Internet so that we cannot find it, there may also be less important classified information. Those who classify information may even classify some completely insignificant data just to use as web trap bait to make us criminals by accident. That is, information that actually have no military relevance at all but is merely said to have it to make the information illegal to read.
Porn is another example. Individual differences in maturity certainly mean that there are people over the age of 18 who can please literally anybody's desires. I used to live right across the street from an animal rights group full of people who were stunted by the vegan diet their parents forced onto them, so I know that there are adults who look like children (not my cup of tea, I prefer more mature stuff, but I know that they exist). So "sick people's desires" cannot explain the existence of illegal porn. But police with the interest of making people criminals by leaving illegal websites up can.
In all of these cases, police have the possibility to plant illegal information into people's computers too.