I think the ability to make that kind law is fair enough but its implementation is often hysterically overdone. Moonshadow's comparison to the fear of witches might be a good example of what is going wrong with the system.
I think we have progress, then. So, whether the subject is 'child porn' or 'spells' or simply 'thought crime'; in your opinion what has actually gone wrong with the system? Is there some kind of rational distinction between justifiable prevention of harm to children and thought crime based upon same? Or is it all just an arbitrary line that we must all respect, once some government body has negotiated the line?
We already agree on what constitutes the basics of a decent society - where we disagree is how to implement it and how far beyond the basics we should go
"Is there some kind of rational distinction between justifiable prevention of harm to children and thought crime based upon same?" - leave children out of the sentence. We prevent infliction of harm on all where we can.
In the UK, there are indeterminate sentences. In the US, there is "3 strikes and you are out." Both address the issue of people who are never going to stop harming others. I am not comfortable with either idea but I have met someone who does spent his life either in jail or terrorising people and I accept that society needs protection.