I don't know how they could discover the bad people online without snooping on the innocent. I also don't know how snooping is done though. There can't be a board of people all looking at what everyone on the Internet says or does. There'd have to be more snoopers than innocent users for that. :)
I've often thought it might be done by servers looking out for certain keywords that are related to crime. Start sending any of those words across the Internet and they might get picked up on and diverted to someone who looks at the whole communication to see if there is anything dodgy going on.
If that is the case, then what we say or transmit across the Internet won't get taken any notice of at all if we don't write/send those words. I wonder if that is the way it works? It could be.
There probably is also a degree of Internet scouring of sites known for being a source of crime, slander, bullying etc. It would be quite right to monitor such sites in my opinion. People go too far under the protection of anonymity. They ought to be held to account for their words and actions just as they would be if they were said or done in life away from the Internet. People expect to be policed and kept an eye on in all other aspects of life to make sure laws don't get broken, so why not on the Internet too?